I'm pretty up to par that to 'Donk Bet' is leading OOP after calling a PFR with a board giving us middle pair or believeing that the board has missed our OPP, am i correct? Are there other forms of 'Donk Bets'?
A donk bet is just leading into the aggressor on a previous street. It doesn't have to be with any particular hand.
Generally, leading into the pre-flop raiser with a hand like middle pair is terrible. You should be check/calling or check/raising most of the time. This way, the preflop raiser will make a continuation bet, and you will get the maximum from the vast majority of his range. Here's an example:
A super tight player raises from the button. For the sake of this example, he can only have 2 hands: AA & AK. You call from the BB with pocket 8's. The flop comes out 2h 3s 9c. If you bet, your opponent will throw away AK and call you (or raise) with AA. So your bet is only being called when you are beaten, and it has a negative expectation. However, if you check, sometimes he will bet AK, and you can get money from both AK & AA.
There are times when you want to lead into the PFR. For example, if you call a raise from the button and are holding Jh 10h] in the big blind. The flop comes out Jc 10s 8c, then you would want to lead into the PFR, for the following reasons:
1) It is unlikely that the pre-flop raiser will c-bet this flop, because it hits the range of a cold caller very hard, and it is quite scary.
2) There are many draws or made hands (like KJ) that the raiser could check back on the flop. You don't want to give a hand like AK or 98 a free card.
3) Leading here allows the preflop raiser to raise you on the flop with draws & other hands you beat. This is a much simpler way of bloating the pot, and it shows less strength than check/raising. Thus, its less likely to scare our opponent away. Plus, bet/3-betting has a larger chance of getting our opponent pot committed with whatever hand he may have.
BTW - I habitually raise donk bets like its my job. So often its a hand like middle pair that wants to "see where they are". So I lie to them. I call with top pair good kicker, and I raise when I have nothing.