RAM - random access memory is what you're looking for.
go to any computer store and ask for it, they'll have it.
things you need to know...
1. to upgrade you need to check what type you currently have in your system. there are different speeds of RAM and different types too... if you can take a sample to the sales person that would be easier for them to know what you actually need. if you get the wrong one your pc will not boot up. you will not lose any data or anything but the system will just not start.
2. if you hear (i think 5 or 7) beeps when you start your machine then the ram is not installed properly or your video card needs to be taken out and put back in.
3. make sure you have an extra slot for the extra ram ... if not then you will need to replace the current one with one that has more capacity. for example 1 chip - 1gb instead of 1 chip 512mb.
4. on laptops you do not need to open the whole laptop ... on the back of the laptop somewhere in the middle you'll see a portion that you can unscrew ... when you open that portion you'll see the extra slot. the size of the ram chip is much smaller in laptops than desktops.
5. sometimes the motherboard does not support more than a certain amount of ram. for example when i upgraded my laptop a 2gb chip did not work but a 1 gb chip worked. the max my board could handle was 2gb in total.
hope that helps.