As many of you know, I used to exclusively play UB. This was until I learned more about bonus whoring and the profits I can make at PP. Now to the subject at hand...
For something to qualify as a donk-bet:
a) Someone else must have bet or raised the flop.
b) A scare card appears on the turn and you lead out with it instead of check-raising.
I'll give a quick example. The flop comes J84, you check, MP bets, LP raises, you call. The turn is another 8. You lead out with a bet. You have just made a donk-bet!
On PP the players are far less rational than UB, so they donk-bet often... but when they do they also usually have the card they're representing! Why would you donk-bet when you hit your hand against aggressors? You could check, let them bet, and then raise to get some extra bets in. People on PP simply do not understand this.
On UB, when someone donk-bets me, I often re-raise them on the turn. As I mentioned... UB players are more rational. If he really had the scare card, he would go with a check-raise to get more money in the pot.
So for those of you new to PP... watch out for the donk-bets and give them more credit than usual. Many of these players don't care about saving bets or winning more bets, they just care about winning pots.