For some reason or another, you may have decided to cash out from a site. Hopefully, you'll find yourself with more money than you deposited - and if so, congratulations and well done. Cashing out, or withdrawing, is usually very straightforward but not often as straightforward as depositing. Perhaps understandably, the sites are very eager to make it easy for you to deposit with them, but may not have gone through all that much trouble making sure it's just as easy to withdraw. Nonetheless, getting your money shouldn't be a major hassle. For the most part, it takes a few days to clear the transaction, before the money appears back in your bank account (if you've deposited using a bank debit card) or in your NETeller account.
Cashing out at PokerStars. Because I deposited using NETeller, the withdrawn amount will automatically return there.
Some sites offer instant NETeller cashouts to the amount that you've deposited, meaning that if I've deposited $500 at Full Tilt Poker using NETeller, the first $500 that I cash out will be instantly transferred back to the account it was deposited from. I know PokerStars and Poker.com offer the same service, and likely quite a few more sites. The reason I bring this up isn't to celebrate the fact that they have this feature, but rather to point out that it's limited to the amount of your first deposit, so that it won't come as a surprise to you later on when you try to withdraw more money than you originally deposited. You will still get your excess money, but it won't be instant - you may have to wait a few days.
You should also be certain to look into what the minimum withdrawal amount is, if there is one, and this I know has been a problem to many: They deposit $50 or so, play for a few weeks and lose a bit of the money. They get tired of the site and want to withdraw and move on - only to find out that the minimum cash out is for $50, meaning that they either need to deposit more money (that they later can withdraw) or they have to try to win back enough to withdraw, or they just plain have to leave the money where it is. Always read the terms and conditions if you're not sure what the rules are, and as with depositing, be sure not to hesitate to contact the site's customer service if you have any questions.
A final point is that some sites may want to verify your identity by asking for a photo/scanned copy of an ID (passport/driver's license or so) and a utility bill (thereby seeing that your address matches what you've stated). I've done this at two times myself and didn't worry much about it, but I know others who feel this is an intrusion of privacy in a sense, and aren't comfortable leaving that kind of information to the poker sites. Either way, you should be aware that some poker sites may require you to verify your identity this way, and they most likely will demand this at the time when you try to cash out; this ties in to what I said about it being easy to deposit but sometimes more complicated to withdraw.