RAM (Random Access Memory) or just memory as its commonly called is always evolving and getting faster and faster while the capacity keeps on getting larger. All computers come with memory and the more you have the better (to a point) and the faster the type of memory you have the better. Keep in mind that you can't just put any kind of memory into your computer. You have to use the type that your motherboard supports. Even though there are many speeds of memory one of the common ways people differentiate between the types is if it is DDR or DDR2 memory.
DDR stands for Double Data Rate memory. DDR memory has double the bandwidth of the older SD-RAM. Newer computers are using the faster version called DDR2. If you are thinking about upgrading, don"t because you cannot use DDR and DDR2 at the same time.
DDR offers data rates of 200/266/333/400 Mbps and DDR2 offers 400/533/667 and DDR2 has double the latency plus a reduced voltage requirement. Higher speeds of DDR 2 memory allow it to have up to 10.6GB per seconds of throughput compared to DDR1 memory which maxed at 6.4GB per second. Other than that there are not a whole lot of performance gains from DDR2 unless you overclock it. When the DDR 667 becomes more common that may change but then again DDR3 will soon be on its way!
Update: DDR3 is here and faster than ever!