There is a good introduction here:
https://en.bitcoin.it/wiki/Introduction
There are private keys and public keys. (By the way, NEVER share your private key with anyone.) You can have many, many pairs of private and public keys. People send money to one of your public keys and then you have access to it with the corresponding private key.
Some recommend that you use a new public key for each transaction and the bitcoin qt client supports that as do many others.
so here's my newbie question. it is my understanding that every bit coin wallet comes with two numbers. a deposit number which you use with transactions; and a withdraw number. my question is what's the withdraw number for? what do you withdraw when you use this number?
