RAM or Random Access Memory, is the amount of short term memory available to transfer data. The higher the amount of RAM the more processes your VPS will be able to handle at the same time.
CPU or Central Processing Unit, is the brain of your VPS, it handles the instructions of computer programs and make sure they do what they need to. More CPU's result in more brains acting as one to get processes completed faster.
Bandwidth refers to the data transfer rate, the amount of data that can be transfered from one point to another in a given time period. This directly effects network performance.