After the flop, there are basically three possible situations:
You have a made hand – that is, your hand is very likely the best at the moment. This hand will often be able to improve even more, but here the number of outs is not as important because your are already in front. You still have to consider that many cards might improve your opponents hand to beat you (for example if you have 2 suited cards on the board, you don’t really want to see another card of the same suit on the turn because it might give somebody a flush).
You hold Kh Qs, the flop is Kd Td 6h. You are likely to hold the best hand unless somebody holds two pair or the dreaded “pocket rockets” (AA). Any Ace as a turn card might give somebody else a higher pair (players play Aces frequently), any Q or J might give somebody a straight, a third diamond card might give someone a flush.
So you are vulnerable. In this situation you must play aggressively to fill the pot and force the weaker draws to give up so that they cannot develop any further to beat you ( see also: raising for protection, later in this document).
You have a draw – a hand that has outs to develop to be the best hand
Example: You hold Kh Jh, the flop comes 9h 5h 3c.
Here you have two overcards which can make top pair/good kicker, but especially the remaining nine hearts give you a flush. The flop may not have helped anybody else. You have 15 outs. Here you want to bet and raise to fill the pot, and you may even convince the opposition that your hand is made and that they should better fold.
The strength of your draw is determined by the number of outs and the probability that it will really develop to the strongest hand if you hit.
You do not have anything – no pocket pair, the flop has missed you, no chance of improvement: your hand is trash, so get rid of it!