In a 4 player game, Player A bets $10. Player B raises $10 (puts in $20 total), Player C calls Player B (puts in $20). Player D at this point can either fold, pay $20 (call player B) or raise $10 to a total of $30. Player A would go next and either call (paying the difference between his $10 bet and the current bet). This would continue until all players either call or fold.