A poker bad beat occurs when an overwhelming favorite loses the hand. Examples of a daily bad beat would include pocket Kings beating pocket Aces with a third King on the river or a player hitting an inside straight on the river to beat a made hand. In both of these situations, the drawing hand would have, at maximum, 4 outs (cards to come to give a winning hand) and less than a 10% chance of winning.
A natural reaction to suffering a poker bad beat is to want to tell the world about it. Players usually want sympathy and understanding from fellow players and reassurance that the loss wasn't their fault. While this is certainly understandable, it is poor poker etiquette to complain about bad beats at the table. Most of the table simply doesn't care as they've experienced the same situation countless times themselves. The only thing you accomplish by crying about bad beats is making yourself as a target.
The overwhelming majority of us have a tough time dealing with bad beats psychologically, at least temporarily. If you think it's a beginner's weakness to be unable to control emotions after a pot has been cruelly taken from victory's grasp, flip on ESPN and listen for those bleeps after a pro busts out of the WSOP on a 2 outer.
The truth is that beats happen because there is an element of luck in poker. A hand that is an overwhelming favorite to take down the pot before the flop, turn, or river is just that: a favorite. Probabilities and odds don't guarantee results.
In fact poker bad beats are needed to keep new players interested and bring fresh money brought to the tables. After all, who would take up the game if they didn't have a chance to beat more experienced players on any given day?
Always keep in mind that skill does prevail over luck in the long term. Mange your bankroll while playing your best and you will be able to ride out the bad beats, which often come in bunches, causing good players to "get stuck" in the short term.
Over a lifetime of poker, you will find that "short term" loses can run for days, weeks and or even months, which can become a really bad year(s).