Memory

The memory type you choose for your computer will depend on your hardware and the memory slots you have available to you. Modern desktops and laptops typically use either DDR, DDR2, or DDR3 memory. Desktop DDR memory uses 184-pins. Desktop DDR2 and DDR3 memory uses 240-pins. For laptops DDR and DDR2 memory comes in 200-pins while laptop DDR3 memory uses 204-pins. You can check the documentation of your computer to see exactly what type of memory you can support. A great way to figure out your memory type is to read the labeling (if available) on the memory you already are using in your computer and match the module type.

What does maximum memory module size mean?

Every memory slot can be thought of as having a "maximum" memory size you can put into it. It's best not to exceed this maximum module size to ensure that the memory you purchase will work as intended. In some cases the maximum size of memory per slot in your system will be specified for you. In other cases the total amount of memory that your computer can support will be specified and you can divide that amount across your slots. For example, if your motherboard can support up to 4GB of memory and you have two slots, the maximum memory size you can use is 2GB in each slot (2GB in each slot X 2 slots = 4GB).

Do I have to install DDR, DDR2, or DDR3 memory in pairs

With DDR1, DDR2, and DDR3 memory you DO NOT have to install your memory in pairs (matching or otherwise). If your motherboard supports it, though,  you may be able to take advantage of certain technologies & performance boosts by installing multiple memory modules (such as Dual Channel or Triple Channel operation, etc.)