1. Most sites will explain this on their websites, at least the good ones will
2. They pay out X dollars of your bonus for every X amount of rake you spend on the site (cash game rake and tourney fees). This may be defined by how many player points you earn (which is the same thing, points are based on how much rake you spend). These numbers and definitions will vary from site to site
3. I HIGHLY suggest before you do anything to make sure the site (or headhunter site) includes a rake back offer, which is good for the lifetime of your account.
4. Depending on where you live the fine folks here can suggest the best sites to play, OBV PokerStars
will always be #1, but if you cant play there it depends on where you live (and this may not necessarily have anything to do with bonuses, bonuses only last so long and are finite - a great place to play (with rakeback) is forever!)