Most online casinos will offer you a bonus of some sort to join their site. These bonuses can vary from an amount matching your initial deposit, repeat deposit bonuses, loyalty rewards, or even a bonus just for signing up to the site. Some sites even offer combinations of these bonuses to ensure you keep coming back for more and carry on playing at their site.
One thing you need to watch out for if you agree to take the bonus on offer is the terms and conditions attached to it. In many cases you need to wager your own money before you move in to playing with the bonus amount, and you will usually need to meet certain minimum wagering criteria before you can withdraw any winnings. In addition you may find that you have to wager on particular games within the casino to meet the terms and conditions and these games may not be games you would normally play.
Bonuses can be a good thing too, of course, if you are clever about how you play the games and the strategies you use. For example, using certain methods it is possible to win up to ten times your deposit plus bonus when playing online Roulette. Now you may find that you need to wager around half that amount on slot machines in order to qualify to withdraw any winnings. So, even if you end up losing everything on slots you will still have half your winnings available to withdraw, which could be quite a chunk more than your original deposit. In practice you would be pretty unlucky to not win a thing on the slots part of this process, so in theory you could still be taking home a reasonable amount.
Of course, you will always have the option to just deposit your own money and play with that, without accepting any bonuses, leaving you free to withdraw any winnings at any time normally allowed by the online casino.