RAM or "Random Access Memory" acts as a buffer between your hard drive and your CPU and is measured in "Gigabytes"
In terms of performance, the more RAM you have, the quicker your desktop will be.
For general office use, around 2-3GB will be more than enough, and again, if you are running any additional applications, the software vendors "System Requirements" will guide you on precisely how much you will need.
Additional note: If your application demands more than 3GB of RAM, you will need to look at installing, or buying a desktop with a 64bit operating system (visit the operating systems guide). The 32bit operating system most desktops ship with can only use a little more than 3GB (around 3.3GB), meaning if you install more, you will not receive any benefit.