Definition of RAM (memory)
Also known as Memory.
Abbreviation for Random Access Memory
Your computer's RAM allows the temporary storage of information that your computer needs to make calculations, run programs, open documents, etc.
Your PC can store temporary data on your hard drive (known as virtual memory) when your RAM memory becomes used completely. The problem with this is it takes much more time for your processor to access data on the hard drive as opposed to the stick of RAM.
On most new PC's, 512 MB of RAM should be sufficient, and 1 GB is the best for graphics/video editing. The most common type is DDR RAM, while RD (Rambus) memory is gaining popularity.
For more information besides this definition of RAM memory, check out these pages:
Types of RAM -