Whenever you run a program, that program is loaded from your hard drive to your RAM memory. Because you RAM is faster, than your hard drive, that makes the program data faster to access for other parts of your PC, like your CPU. When you turn your PC off everything on the RAM is wiped clean.
How much RAM do you need?
If you are running a large program or many programs you might not have enough memory. In that case your PC has to swap parts of the data to and from the hard drive as they are needed(swapping). This slows down your computer system greatly. Currently I would recommend 1-2 GB RAM, depending on your needs. If you have less you might look into upgrading.
My computer system is running slow, is it the RAM?
You can easily test for this. Run the programs that you usually do until your PC starts responding sluggishly.
XP:
press ctrl+alt+del
Click on the performance tab.
Look for "physical memory".
Then underneath that, "available".
If the number is less than 10,000 the problem is lack of memory.
Vista:
Vista uses memory differently, so you can't check for this in Vista. With Vista you should have at least 1 GB of RAM.
What type of RAM does my PC use?
There are several types of RAM(DDR1,DDR2,DDR3...). Your motherboard will only support some of these. You can find out which ones are supported in your motherboard book.
This is a picture of a DDR2 RAM DIMM
The picture shows 3 RAM DIMMs on a motherboard.
DDR3 is fairly new, your PC probably uses DDR1 or DDR2.
RAM speed is measured in MHz. The higher the number the faster the RAM.
