Some online casinos insist that once you signed up and made your first deposit, you either claim your bonus through a link on the website's “Promotions” page, or you can email the online casino support team to request the bonus
In the past, when you claimed a casino bonus, the bonus funds and real money were combined into one; only once you'd completed the wagering requirements were you able to withdraw money.
Today, however, due to new regulations brought in from the UK's Gambling Commission, it's no longer possible for bonuses to be combined like this; your cash balance and your bonus funds must be separate so you can withdraw your cash balance at any time. This is what's known as a parachute bonus.
What you are primarily interested in is finding out the wagering requirements and the allowed games. These two factors will probably make your decision for you, but whilst browsing through the T&C's of a casino bonus, there are a couple of other things to look out for. You'll need to know whether you are actually “eligible” to claim the bonus.