Definition of preflop
preflop means "before the flop". All poker hands start at preflop. The dealer asks the blinds to put in their respective blind's, then deals the cards to all players. When all players have received their cards, round preflop will begin. The player in the Under The Gun (UTG) position will have the floor. He will have the option to call (limped) the equivalent of the big blind or raise. Until all players have invested the same amount of preflop, the action will continue. Let's say the UTG player raises to $15, each player will have to fold if he doesn't want to call, pay the $15 if he wants to go to a flop or raise again.
There is not always a flop.
As mentioned above, all poker hands have preflop action, but not all poker hands have action after the flop. A classic case is when the action is folded (all players fold successively) to the button that raises. At this point, the blinds must pay the raise or raise in turn if they want the action to continue. If both blinds fold, the button will win the jackpot and there will be no action postflop.