What is RAM?
Random Access Memory (RAM) provides space for your computer to read and write data to be accessed by the CPU (central processing unit). When people refer to a computer's memory, they usually mean its RAM.
If you add more RAM to your computer, you reduce the number of times your CPU must read data from your hard disk. This usually allows your computer to work considerably faster, as RAM is many times faster than a hard disk.
RAM is volatile, so data stored in RAM stays there only as long as your computer is running. As soon as you turn the computer off, the data stored in RAM disappears.
When you turn your computer on again, your computer's boot firmware (called BIOS on a PC) uses instructions stored semi-permanently in ROM chips to read your operating system and related files from the disk and load them back into RAM.
(find more info about RAM here:http://www.kb.iu.edu/data/ahty.html)
When choosing RAM, the main thing you want to look at is the type of RAM. The different types include:
SDR, DDR, DDR2, and DDR3.
If you know which one your RAM is, then you're set, but if you don't this website will certainly help you out:
http://www.crucial.com/ There are two tools on this website you can use. Crucial Memory Advisor™ tool, which lets you manually choose your computer, by selecting manufacturer, then product line, and finally model number. If you are unable to complete this form, there is a some software you can download which will scan your system to tell you what kind of RAM you have. You can access this feature by clicking "SCAN MY SYSTEM" or visiting this link: http://www.crucial.com/systemscanner/index.aspx
Another thing to keep in mind is the speed of the RAM. If you're getting two sticks and one has a lower speed than the other, the one with the higher speed will "clock down" to match the speed of the slower one. Also remember, there is a difference between laptop and PC RAM, laptops are about half the size of PC RAM.
This is the basic information you need to know when purchasing RAM, hope I helped