An action/adventure/all types of speculative fiction/mystery/dramedy/crazy show (2004-2010) created by Jeffrey Lieber, J. J. Abrams and Damon Lindelof, and show-run by Lindelof and Carlton Cuse.
On September 22nd, 2004, Oceanic Flight 815 breaks up in midair and crashes on a tropical island. Only forty-eight passengers somehow survive. Unfortunately, this is no ordinary island they've crashed on. To start with, there's a polar bear roaming about somehow as well as something in the jungle which is capable of uprooting trees. This monster mutilates the pilot, but not before the pilot reveals that the plane was already a thousand miles off course when it crashed, which means the odds of rescue are pretty much nil.
The survivors must learn to work together if they want to survive in this strange and hostile environment. This isn't easy, mainly because the most prominent characters are so utterly screwed up. All of them have something they're hiding in their pasts. There's the seemingly nice woman who's actually a fugitive who was being brought to trial. There's the one-hit wonder ex-rockstar junkie. There's the former Iraqi government torturer who's searching for the woman he loves. And so on.
Their backstories are revealed in flashbacks, with each episode tending to focus on a specific character. In general Anachronic Order is also expected, both with the story of the island and anything before and after the island. If there is a method of skewing the audience's perception of events by rearranging the order of the scenes, LOST has used it.
As the show goes on, more and more questions arise as the secrets of the island are slowly uncovered. The island seems to have magical properties as well as a unique abundance of super-powerful electromagnetism. Furthermore, flashbacks reveal more and more connections between the characters' pasts as if to suggest that it may have been more than coincidence that this specific group of people was all on Flight 815 together with each other.
The show was famous at the time for its high production values (the pilot alone cost $14 million and led to the firing of a ABC executive for greenlighting such an expensive endeavor), inhumanly dense plotting, a tendency to raise more questions than it can answer, and somehow remaining a smash hit despite all these "hurdles". The show also heavily involved its fandom in the storytelling process through the parallel Alternate Reality Game The LOST Experience. The success of Lost inspired network TV to commission a slew of imitators (The Event, Jericho, Surface, Terra Nova, Invasion!, FlashForward (2009), Revolution, and J. J. Abrams's follow up show Alcatraz), all serious and densely plotted serialized dramas based around a central mystery and tinged with a sci-fi edge. Unfortunately most of these shows have failed to recapture the lightning and almost all of them were cancelled after one or two seasons. Honorable mention goes to Prison Break, while not a sci-fi show, it was the first out of the gate to capitalize on TV audience's newfound love for serialized dramas, it was also arguably the most critically-acclaimed and successful of the Lost-clones, running for four seasons.
Lostpedia (which the producers themselves occasionally namecheck in DVD commentaries for its expanse of knowledge) has exhaustively catalogued (almost) every aspect of Lost. If you want insight into the show or just want to learn some random statistics, it's definitely worth checking out.
Recaps and summation of the show here.
God loves you as He loved Jacob.