why cant someone just "use" my bitcoin wallet address?
They can. And that would be awesome, since they'd be sending you money.
i downloaded bitcoin software from bitcoin.org and it gave me a wallet address automatically - im pretty sure i diddnt even put a password into the address
im guessing when you send bitcoins you have to know their address to send it to
so why cant i just use someone elses bitcoin address and transfer all their bitcoins to my account or something?
Because you only have the address, not the information needed to claim coins sent to that address.
i admit im a total newb i have no idea what im doing but i cant seem to find answers to how this "wallet" thing works - is it tied to my IP maybe?
No, it's not tied to your IP, it's tied to your wallet.
When your client generates an address, it first generates a key (called a 'private key'). It then generates a lock that only your key fits (which is not exactly the same a public key, but very similar). The address is the lock
, not the key. You give other people the lock so they can send coins that you have the key to. You never reveal that key to anyone, and it remains in your wallet.
The mathematics that make all this possible are quite amazing actually. If you want to learn about them, I'd start with this:http://en.wikipedia.org/wiki/Digital_Signature_Algorithm
It's not the same algorithm that bitcoin uses, but it's very similar and much easier to understand.