Almost all offices tries to get new clients with help of entry bonus of first deposit. Betting office gives you a welcome bonus either immediately or after betting some money. Entry bonus may be a free bet (risk-free bet). This means that the office gives you a free bet (some just in case you loose your first bet).
Free bets are also given as a bonus when football World or European cup is played. Betting bonuses are pretty much the same in most offices. Usually you get an entry bonus, usually 100% of your first deposit (with some limit of course). That´s the way it goes at Betathome and Bet365.
Sometimes you get a free bonus just for signing up. If the ofice also runs a casino, you will be probably able to use your bonus even there. There are also special bonus for current clients. For example if you don´t place a bet for some time, the office will offer you for example free bet or better odds. The office gets nothing of that, but gets you back to the game.
Just play regularly and the bonuses will come. If you give the office your friend who will open an account, you will get a bonus at most of offices. These bonuses are very worthy and you won´t ever get it in traditional bookmaker point.