You've definitely seen them, if not heard of them. But cliff-hangers are the bread and butter of television executives. They're what keep viewers tuning in each week, each month - heck each season, to a particular show. Cliff-hangers in short, are those little surprises that tv shows throw in so that you'll continue watching to find out. There's several variety of cliff-hangers, but the most obvious are what I not-so-cleverly refer to as the Major and Minor cliff-hangers.
The minor cliff hangers are what get you initially interested in a show and help to further the plot along within the current episode. It's the dilemma that comes up after a few minutes, the small choice or problem that the characters have to deal with in that particular episode. Let's take Heroes for example.
In the Heroes episode Distractions, there are several minor cliff-hangers - the first of which comes when alter-ego Niki/Jessica (Ali Larter) manages to break free of her prison cell and escape. Almost immediately this takes us in to an ad-break. And therein lies the reason for this, to keep the viewers interest long enough that they will sit through the ads and continue watching, rather then flipping the channel to something entirely different. Heroes makes moderate usage of this, and it constantly throws in minor problems that will quickly be resolved after the ad breaks are done. Things such as how Hiro will get access to a room. After the ad-break it becomes clear that he will manipulate time to enter through, and so forth. Minor cliff-hangers are always solved shortly once the show comes from an ad-break - only to be replaced with a new minor cliff-hanger (in this episode, he get's kidnapped by some mysterious figure) --> Back from the ad-break, and it's revealed it was his father. They repeat this ad-nauseum, until.. the last 3-5 minutes of the show.
At about 3-5 minutes at the end of most shows, we end up with a plot roller-coaster with questions earlier asked answered. But near the end, a major problem needs to be realized. Something substantial. In Heroes, Lost and Prison Break you end up with an amazing discovery that is yet unrealized to the characters - but of course known to us as the 3rd party (as viewers we see the larger and bigger picture). In the particular Heroes episode, it was the fact that the cheerleader's (Hayden Panettiere) father was in actuality another major character.
The most interesting aspect of cliff-hangers is that the more readily a tv show uses them, the more successful they seem to be. Lost which is celebrated as one of the most successful TV shows ever to grace prime time television - is possibly the most heavy user of cliff hangers. The show uses somewhat major cliff-hangers to actively keep the audience glued, only to do this again and again at every ad break. It seems that this sort of bait-and-hook tactic would get old, but for some reason with Lost it doesn't seem so. But that may have a lot more to do with the fact that unlike Heroes and Prison Break, which have stuck to a more realistic schedules, Lost appears to be going on intentional hiatuses so as to pace the show and ensure that viewers don't get tired with the antics of Jack and the lostees.