What is the Island?

Since season one of the show the only thing on everybody's mind was the island. What the bloody hell was it? For a while people thought the whole show was Hurley's dream and the island was just a figment of his imagination. Other people thought the island was purgatory and that everybody died when the plane crashed (they weren't too far off with this guess). The island is something a little more complicated than that.

The man in black examines the bottle and cork metaphor

            The island contains strong electromagnetic forces contained within it, like a bottle containing fluid (the comparison they make in season 6). When the cork comes off the bottle, fluid comes out. The island has a "cork" that contains the electromagnetism. A chink in the armor. If the cork comes off, the radiation is uncontained and all hell breaks loose. This is why Desmond was put in the hatch, to push the button every  108 minutes to contain the radiation.

            The reason the radiation is bad is because it contains the smoke monster on the island. If the radiation is let loose then the smoke monster can leave the island and do bad things elsewhere, I don't know (this is where the story got a little farfetched).

The physical cork as seen in the finale that keeps the
electromagnetism from leaving the island. (Who'd have
 thought they were actually talking about a REAL cork?)  

The smoke monster in his human form (left) and
his brother, Jacob (right), leader of the island.

            Like any epic saga (which is what I guess you could call Lost) there is a good to every bad side. As the smoke monster is evil, the white knight is Jacob, the savior of the island. He keeps the smoke monster from leaving the island. The feud between them started hundreds of years earlier and comes to an end in the finale.

            In addition to all of this other mumbo jumbo, the island can move! It can relocate itself as long as someone turns a giant wheel. The only rule is the person who turns the wheel is not allowed back on the island.

            So that's all the island really is. It's a container for electromagnetism. This is just in its physical state. The island has a much larger, spiritual purpose in the lives of the characters which is explained in the finale.