Random Access Memory is the function that process active programs and data. It allows the computer to store data and programs temporarily. The more memory you have, the more programs you can run simultaneously and the more data can be held for faster access by your machine. RAM memory does not retain your data after shutdown.

When buying RAM memory, you must purchase a memory that is compatible with your computer. Your motherboard could either have DIMMs (dual inline memory modules) or RIMMs (rambus inline memory modules) or SIMMs (single inline memory modules) slots which are different in size. and style. DIMMS are available in both 168-pin and 184-pin connection. RIMMS are available 184-pin connection.
There are various types of memory such as SDRAM, DDR SDRAM, and RDRAM.

SDRAM Synchronous DRAM is previous version of DDR SDRAM compatible. It has sub groups of PC66, PC100, PC133, and PC150 SDRAM. SDRAM is available in DIMM form.
DDR SDRAM Double data rate SDRAM is enhanced newer version of SDRAM. It's available in PC1600,PC2100, PC2700, & PC3200. DDR SDRAM is available in DIMMs form.
RDRAM Rambus DRAM is another popular type of RAM memory which is normally available in RIMMs form.
The best advice of all is to read your motherboard documentation and purchased exact ram memory type mentioned. If you are upgrading the memory, take out the pre-installed ram and buy exact type.