Reasons to bet/fold:
<ul type="square">[*]There are alot of hands that will pay you off.[*]You don't think villian will bluff-raise.[*]
worse hands will check behind, but better hands will bet, which you'll have to call anyway.[*]There are multiple opponents calling you down where one may have improved to beat you, but if not, there's alot of value in a bet.[/list]
Reasons to check/call:
<ul type="square">[*]There doesn't seem to be too many hands that will pay off.[*]To induce a bluff from a hand that would otherwise check behind. [*]You think there's a good chance you'll get raised by a better hand, but you can't fold.[*]When you think villian will bet most of his hands when he's behind and checked to, and he will raise his better hands when you bet. [/list]
There are other reasons, but this is a pretty good basis. You need to weigh all these ideas, because it's rarely clear cut.