The Rules

I've played several unrated games before and I've always wondered that what if I broke a rule and nobody knew about it. For example, are you allowed to castle during or through check? I really doubt the rule of: if it is 25 moves after the last piece besides the King is taken or he is unable to move, is it a stalemate? I've played people who follow different rules than what I believe is right and I am just wondering whose right?