Memory or Random Access Memory (RAM) slots into the motherboard and is where all of the information being used by the processor is stored. For example when you start a program, open a picture or play an MP3, it is moved from the hard drive into memory so that the processor can access all the information related to it. It does this because memory can be accessed many times quicker than a Hard Drive. When the computer is switched off, the information stored in memory is lost.

Having more memory means you can run more programs at the same time without the computer slowing down. This is because when your computer runs out of memory, it starts using the Hard Disk as 'virtual memory', which is extremely slow. If you've ever had lots of windows and programs open at once and suddenly found your computer becomes unresponsive even though it appears to be doing very little then you will understand how frustrating this can be.

For a basic PC, 1GB is a good starting point and for those running Windows Vista 2GB is more appropriate. If you intend to use more memory intensive programs like games or design then 4GB + is recommended.

Faster Memory can make your computer quicker, but only if your processor and FSB are fast enough to make use of it. As a rough guide try to get memory that is around 3/4 of your FSB speed as beyond this there will be very little performance difference.