The island is another land like Narnia in C.S. Lewis's Chronicles of Narnia. One can enter it in a number of ways, but one cannot enter and leave at will. The Island itself calls you or sends you back. Some of the people of the Island go back and forth between Earth and the Island, others are part of the Island culture and never travel between earth and the Island.
Time on the Island is different than time as we know it. In Narnia, when you leave Earth, you could spend years in Narnia, and when you return to Earth, you have only been gone hours. Likewise with the Island, in the flash forward (Ep: 3x22, Through the Looking Glass), Jack and Kate could have returned to Earth at a time before the crash of Oceanic Flight 815. Like with Narnia and the Magician's Nephew book, they will, I believe, return to the Island.
Many on Earth do know of the Island. What happens on the Island might affect what happens on Earth -- Jacob -- could be evil, could be good -- we will see.
Share your thoughts on Lost fan Susan's theories in the The Island is Another Land blog post.
