Its very simple. A value bet
is a bet you are making because you are pretty certain that you have a better hand than your opponent and you are betting for value. (money=value) You are trying to get the most money(value) out of your hand.(because it is most likely the best hand)
is the opposite. You are pretty sure your hand is the worst but you are betting to make villain fold.(either by using the board or giving him bad pot odds
So every time you make a bet , make it clear to yourself if you are betting for VALUE or as a BLUFF.
A semi bluff
is when you are betting with a hand that might not be the best hand but has many outs to improve to the best hand.(such as an open ended straight draw with a flush draw. ...etc etc.)
Also keep in mind that when you are betting for value, you must not make the bets too big because if you fold out the hand, you lose the value of making worse hands call. Bet sizing is important. It is also important to put villain on a range . the better the hand you may think he has, the higher value you can get. Just make sure your not value betting for villain. Meaning betting into a better hand than yourself.