1) So as long as you remove your bitcoins from the exchange immediately then you are still in possession of your bitcons, correct?
2) To get USD in return for bitcoins I assume you reverse the process and send your bitcoins to an exchange?
3) Is there another purpose for these exchanges, for instance do some people store their bitcoins there for weeks/months?
I'm confused what is the benefit of storing your bitcoins at an exchange? I would just store my private key in my safe at home and a few other hiding places outside of my digital harddrives.
1) Yes. If you remove them from the exchange to your own wallet, then the exchange can get hit by a meteor and it won't affect you.
2) You can certainly do it this way, yes. But there are also other options, such as face to face trade via sites like localbitcoins.com
3) Trading between crypto currencies is the major function of an exchange. And you would store the coins at the exchange for as long as you wished to trade. After you;ve made your profits or your losses, you can always just transfer out to your wallet again. The disadvantage of keeping coins on exchanges longterm is that you are then exposed to the same risks as the exchange - i.e. natural disaster, theft, scam, insolvency, fund seizure etc etc. See the ongoing tale of MtGox for an example.
Hope that helps,
Rit.
Agree.