the deposit bonuses are usually worded 100% match up to 600$. i.e deposit 100$ , you get 100$ of bonus available to you. Sometimes that bonus comes in 2,5 or 10$ chunks after you have earnt a certain number of points other times you have to clear the total number of points before you get the bonus.
Sometimes you can get free money given to you to open an account with a particular pokersite.They are limited (non-existant ?) for USA players but are available for players around the world.You can't just get the money and withdraw it though. You will have to play a lot of hands , earn a certain number of points , which means that the site has earnt a certain amount of rake to cover that cash that has been given to you before you will be alowed to withdraw it.