The short and perhaps the most efficient non-technical answer is that CrossFire is a high-performance PC gaming graphics platform made possible by the use of multiple graphics cards coupled with a CrossFire enabled motherboard with a single CPU, in order to increase graphics performance and quality. So, in layman terms you can essentially run applications like high-end games at a much higher framerate than it is possible with a single graphics card.
Previously called CrossFire, AMD CrossFireX (as it is called today), refers to a brand name for the multi-GPU system by Advanced Micro Devices which was in fact originally developed by ATI. Today, up to four GPUs can be used on a single computer, with the possibility of enhancing graphics performance by up to 4X that of just one GPU, but there are limitations as we discuss below.
First Generation CrossFire
CrossFire was originally introduced to the public in 2005. The setup required a CrossFire-enabled motherboard along with a pair of Radeon PCI Express ATI Crossfire ready graphics cards. There were a couple of compatible cards which included the Radeon X800s series the X850s, X1900s and X1800s. All these cards came in a CrossFire and regular edition. The "Master" capability, was in the form of five extra image compositing graphics chips, which could combine the output from both cards. So, a user would have to purchase a Master card and then it would have to be paired with a regular card of the very same series. The Master card came with a DVI Y-dongle, that would be plugged into the DVI ports of both cards and then into the monitor. The dongle worked as the link between both the cards ("master" and regular), which would send the incomplete images between both cards and then finally be sent to the monitor post processing.
The low end (cheap) Radeon x1300 and x1600s cards do not have a CrossFire Edition, but are rather enabled via a software interface, that reroutes their communication via the PCIe slots on the motherboard. Though FireGL cards do not yet take advantage of the CrossFire configuration.
The latest generation of AMD CrossFire cards (AKA second generation)
The second generation no longer requires a so-called "master" card. Models 3200 onwards can run two normal cards (2-way crossfire) in a high-end Crossfire setup, on a regular PCIe bus. This works a lot like the X1300, but the X3200 has a lower latency along with higher speed communication between both cards. Even though performance took a slight but noticeable hit, it was hailed as being an overall improvement since "master" cards were always expensive and so was a deterrent for people with mid-range budgets. Plus, because of demand finding a "master" card become a rarity with some selling these cards for almost twice the retail price on eBay and other similar websites.
Even though the X3200 chipset is capable of CrossFire via the PCIe bus, for almost every series older than the X190ss, the accompanying drivers are still not available for this method, namely the X1800 series. Though ATI has said that there will be future versions of the Catalyst suite which will have what is needed for the X1800 dongle-less CrossFire though an exact date is yet to be mentioned.
What is CrossfireX?
Crossfire-X technology allows you to connect two or more of the same family GPUs together for combined performance. The great thing about Crossfire X is that you do not slow down the faster GPU's clock rate when running it in conjunction with other graphics processors. So for example, if you run a Radeon 7950 and a 7870 together in CrossfireX configuration, that is fine. This differs from Crossfire and SLI which required you to pair the same GPUs together.
What is a crossfire bridge and do I need one?
A crossfire bridge is simply a short cable that would run between two or more graphics cards so that the xfire connection could take place. Whether or not you'll need the bridge cable between your cards depends on the GPUs and card combinations you're putting together. For example, most 280x series or older still require a crossfire bridge connection. However, the 290 and 290x series cards have bridge-less crossfire support.
Motherboard support
To build a system with crossfire support you first need a motherboard that supports CrossFire or Crossfire X at dual x16, x8 or x16x4 speeds. Dual x8 is perhaps the best when it comes to performance mostly on par with x16 in a number of games. Though x16x4 is largely different. On crossfire motherboards where there is a PCI-E bus, there is a big difference between x16x4 compared to 8x and x16. But on PCI-E 2.0/3.0 boards the difference is not too big though the performance improvements or the lack thereof may be noticeable in a couple of games especially those played at 1080p and beyond. However, most experts recommend either dual x16 or an x8 setup for multiGPU rigs. Make sure your crossfire video card and motherboard are compatible with each other and you'll be set.
Limitations of AMD CrossFire
One of the biggest limitations of the CrossFire platform is that the slave graphics card and the Master has to be from the same family. So, this means that buyers will need to buy two of the same types of cards, which would severely limit the possibilities of upgrading a couple of years later with faster and newer model / family of a CrossFire card since both cards will need to be replaced. This is despite the fact that subsequent versions like the X1800 Master cards had two sets of composting chips along with a double density dual link DVI Y type dongle which doubled the overall bandwidth between the two cards, and effectively raising the resolution, a drawback that originally bugged CRT monitor users. Though the widespread implementation of LCD monitors has done away with the refresh rate issue.
The other more notable drawback of Crossfire is that cards with two different memory versions cannot be used. So, you cannot pair the GeForce 7800GTX 256 MB with a 512 MB version. It cannot even be paired with the 1GB version unless you use some sort of software which ATI obviously does not recommend. Though if by some miracle you were able to run these graphics cards in a CrossFire setup with some degree of stability, then you"ll discover that the model with the highest amount of RAM will be downgraded to match that of the card with the lowest memory. So, if the 512 MB card was paired with the 1GB version, for instance, the 1GB version will be downgraded to 512 MB, which pretty much defeats the purposes because there are instances where performance may take a hit because of it.
Conclusion
Though many of the latest games are compatible with CrossFire, there are some that are not. There are also games that end up performing poorer on a CrossFire setup. That said the only way to know is to do a bit of research to find the right rig and CrossFire setup prior to proceeding. The same goes if you"re going to use a CrossFire enabled rig for video editing or graphics design. Apart from that a CrossFire setup makes a lot more sense today than it did five years ago. Ease of use and wider compatibility are two of its biggest selling points.
List of NVIDIA and AMD/ATI MultiGPU compatible Graphic Cards: (All of them are PCI-EXPRESS cards,AGP cards don't support any kind of multiGPU setups)NVIDIA cards(Desktop models)
Geforce TITAN Family:
NVIDIA GEFORCE GTX TITAN X
NVIDIA GEFORCE GTX TITAN Z
NVIDIA GEFORCE GTX TITAN BLACK
NVIDIA GEFORCE GTX TITAN
Geforce GTX 900 Family:
NVIDIA GeForce GTX 980
NVIDIA GeForce GTX 970
NVIDIA GeForce GTX 960
Geforce GTX 700 Family:
NVIDIA GEFORCE GTX 780Ti
NVIDIA GEFORCE GTX 780
NVIDIA GEFORCE GTX 770
NVIDIA GEFORCE GTX 760
Geforce GTX 600 Family:
NVIDIA GEFORCE GTX 680
NVIDIA GEFORCE GTX 670
NVIDIA GEFORCE GTX 660
NVIDIA GEFORCE GTX 660ti
NVIDIA GEFORCE GTX 650ti BOOST (must be the boost model, normal 650/650ti's are NOT SLI ready)
Geforce GTX 500 Family:
NVIDIA GEFORCE GTX 590(1 PCB*)
NVIDIA GEFORCE GTX 580
NVIDIA GEFORCE GTX 570
NVIDIA GEFORCE GTX 560Ti
NVIDIA GEFORCE GTX 560
NVIDIA GEFORCE GTX 550Ti
NVIDIA GeForce GT 545 DDR3
Geforce 400 Family:
NVIDIA GEFORCE GTX 480
NVIDIA GEFORCE GTX 470
NVIDIA GEFORCE GTX 465
NVIDIA GEFORCE GTX 460
NVIDIA GEFORCE GTS 450
Geforce 200 Family:
NVIDIA GEFORCE GTX 295
NVIDIA GEFORCE GTX 285
NVIDIA GEFORCE GTX 280
NVIDIA GEFORCE GTX 275
NVIDIA GEFORCE GTX 260 Core 216(You can pair it with the Core 192 version.)
NVIDIA GEFORCE GTX 260 Core 192
NVIDIA GEFORCE GTX 250(You can pair it with 9800GTX+ in SLI configuration,however make sure that both cards have the same amount of memory)
Geforce 100 Family:
GeForce GTS 150
GeForce GT 130
GeForce GT 120
Geforce 9 Family:
NVIDIA GEFORCE 9800GX2 (2 PCB's)
NVIDIA GEFORCE 9800GTX+(You can pair it with a Nvidia 9800GTX in SLI configuration,but it's recommended to use the 9800GTX+ card in the first PCI-E 16x slot.)
NVIDIA GEFORCE 9800GTX
NVIDIA GEFORCE 9800GT
NVIDIA GEFORCE 9600GT
NVIDIA GEFORCE 9600GSO
NVIDIA GEFORCE 9500GT
NVIDIA GEFORCE 9400GT
Geforce 8 Family:
NVIDIA GEFORCE 8800ULTRA
NVIDIA GEFORCE 8800GTX
NVIDIA GEFORCE 8800GTS 512
NVIDIA GEFORCE 8800GT
NVIDIA GEFORCE 8800GTS 640
NVIDIA GEFORCE 8800GTS 320
NVIDIA GEFORCE 8800GS
NVIDIA GEFORCE 8600GTS
NVIDIA GEFORCE 8600GT
NVIDIA GEFORCE 8500GT
NVIDIA GEFORCE 8400GS
Geforce 7 Family:
NVIDIA GEFORCE 7950GX2 (2 PCB's)
NVIDIA GEFORCE 7900GTX
NVIDIA GEFORCE 7950GT
NVIDIA GEFORCE 7900GT
NVIDIA GEFORCE 7900GS
NVIDIA GEFORCE 7800GTX 512
NVIDIA GEFORCE 7800GTX 256
NVIDIA GEFORCE 7800GT
NVIDIA GEFORCE 7600GT
NVIDIA GEFORCE 7600GS
NVIDIA GEFORCE 7300GT
NVIDIA GEFORCE 7300GS
NVIDIA GEFORCE 7300LE
Geforce 6 Family:
NVIDIA GEFORCE 6800ULTRA
NVIDIA GEFORCE 6800GT
NVIDIA GEFORCE 6800GS
NVIDIA GEFORCE 6800
NVIDIA GEFORCE 6800LE
NVIDIA GEFORCE 6600GT
NVIDIA GEFORCE 6600
NVIDIA GEFORCE 6600LE
NVIDIA cards(Notebook models)
Geforce 900M Family:
GeForce GTX 980M
GeForce GTX 970M
GeForce GTX 965M
Geforce 800M Family:
NVIDIA GEFORCE GTX 880M
NVIDIA GEFORCE GTX 870M
NVIDIA GEFORCE GTX 860M
Geforce 700M Family:
NVIDIA GEFORCE GTX 780M
NVIDIA GEFORCE GTX 770M
NVIDIA GEFORCE GTX 765M
NVIDIA GEFORCE GTX 760M
NVIDIA GEFORCE GTX 750M
Geforce 600M and MX Family:
GeForce GTX 680MX
GeForce GTX 660M
GeForce GTX 670MX
GeForce GTX 675MX
GeForce GTX 670M
GeForce GTX 675M
Geforce 500M Family:
NVIDIA GEFORCE GTX 580M
NVIDIA GEFORCE GTX 570M
NVIDIA GEFORCE GTX 560M
Geforce 400M Family:
NVIDIA GEFORCE GTX 485M
NVIDIA GEFORCE GTX 480M
NVIDIA GEFORCE GTX 470M
NVIDIA GEFORCE GTX 460M
Geforce 200M Family:
NVIDIA GEFORCE GTX 280M
NVIDIA GEFORCE GTX 260M
Geforce 100M series:
NVIDIA GEFORCE GTS 160M
NVIDIA GEFORCE GTS 150M
NVIDIA GEFORCE GTS 130M
Geforce 9 Family:
NVIDIA GEFORCE 9800MGTX
NVIDIA GEFORCE 9800MGT
NVIDIA GEFORCE 9800MGTS
Geforce 8 Family:
NVIDIA GEFORCE 8800MGTX
NVIDIA GEFORCE 8700MGT
NVIDIA GEFORCE 8600MGT
Geforce 7 Family:
NVIDIA GEFORCE 7950GTX
NVIDIA GEFORCE 7900GTX
NVIDIA GEFORCE 7800GTX
AMD cards(Desktop models)
AMD Radeon R9 Family:
AMD R9 295X2
AMD R9 290X
AMD R9 290
AMD R9 280X
AMD R9 280
AMD R9 270X
AMD R9 270
AMD Radeon R7 Family:
AMD R7 265
AMD R7 260X
AMD R7 260
AMD R7 250X
AMD R7 250
AMD R7 240
AMD HD 7xxx Family:
AMD HD 7970
AMDI HD 7950
AMD HD 7870
AMD HD 7850
AMD HD 7770
AMD HD 7750
AMD HD 6xxx Family:
AMD HD 6990(1 PCB)
AMD HD 6970
AMD HD 6950
AMD HD 6870
AMD HD 6850
AMD HD 6670
AMD HD 6570
AMD HD 5xxx Family:
AMD HD 5970
AMD HD 5870 Eyefinity 6 edition(Can be used in CrossFire with the non-Eyefinity 6 version)
AMD HD 5870
AMD HD 5850
AMD HD 5830
AMD HD 5770
AMD HD 5750
AMD HD 5670
AMD HD 5570
AMD HD 5550
AMD HD 5450
AMD HD 4xxx Family:
AMD HD 4890
AMD HD 4870X2(1 PCB)
AMD HD 4870
AMD HD 4850
AMD HD 4830
AMD HD 4770
AMD HD 4750
AMD HD 4670
AMD HD 4650
AMD HD 4550
AMD HD 4350
AMDHD 3xxx Family:
AMD HD 3870X2(Read the extra information about HD 5970,its the same for HD 3870X2)
AMD HD3870
AMD HD 3850
AMD HD 3830
AMD HD 3650
AMD HD 3470
AMD HD 3450
AMD HD 2xxx Family:
AMD HD 2900XT
AMD HD 2900PRO
AMD HD 2900GT
AMD HD 2600XT
AMD HD 2600 PRO
AMD HD 2400XT
AMD HD 2400 PRO
AMD X19xx Family:
AMD X1950XTX
AMD X1950PRO
AMD X1900XTX
AMD X1900XT
AMD X1900GT(Unlike X1900XT and 1900XTX which require a Master card to run in CrossFire,this card can be paired with another X1900GT card)
AMD X1xxx Family:
AMD X1800XT
AMD X1800GTO
AMD X1800XL
AMD X1650 PRO
AMD X1650XT
AMD X1600XT
AMD X1600PRO
AMD X1550
AMD X1300XT
AMD X1300PRO
AMD X1300
AMD X850 Family:
AMD X850XT-PE
AMD X850XT
AMD X850 PRO
AMD Cards(Notebook models)
AMD HD 6xxx Family:
AMD HD 6970M
AMD HD 5xx Family:
AMD HD 5870M
AMD HD 4xxx Family:
AMD HD 4870M
AMD HD 3xxx Family:
AMD HD 3870M
* PCB stands for Printed Circuit Board, its the hard "plastic-like" colored part in a graphics card/motherboard.
