After missing a couple of episodes and getting lost, I wasn't able to finish the series when it was first run and now that I've heard about the train wreck that the series turned out to be, I'm not too unhappy about missing out. I am curious about something I did see though. Can somebody please tell me what the black cloud that weaved its way through the jungle in season 1 was supposed to be? It seemed sentient and reactionary. Was it ever explained? (I've looked at a couple of websites with differing suggestions like: It's a monster and It's Death. They don't appear to have a common canon answer)
Massive spoilers for the ending of Lost. I'm not going to bother with spoiler tags here.
The Smoke Monster is the adversary and twin brother of Jacob. Jacob is the guardian of the island, and brought the survivors to the island. The Smoke Monster is able to appear as different dead people, in his original form he's just referred to as the Man in Black. In their roles on the island, both Jacob and the Man in Black are immortal, and have been there for millenia. Throughout the series, he uses his smoke monster form and his ability to appear as dead people to manipulate the survivors, especially Locke. His goal was to get off the island, but to do so required killing Jacob, which he accomplished via manipulating Ben Linus. His plan was then foiled by Jack and Kate who were able to turn him into a mortal and kill him.
Massive, in-detail information can be found on his Lostpedia page, although let me know if there is more detail you're looking for.