The Lake of Fire is where Satan, the Antichrist, the False Prophet, and all who followed them, spend eternity after Jesus Christ's return. It is a lake of burning sulfur (the King James Version of the Bible refers to the lake's contents as "fire and brimstone") that is considered a place of unending suffering. Scripture also says that Death and Hell are cast into the Lake of Fire.
Doctrines concerning the Lake of Fire
Opinions about the fate of those who are headed for the Lake of Fire are split among Bible eschatology students. Some believe in the doctrine of annihilationism, where those who go into the Lake of Fire are simply consumed and cease to exist, while others believe that the Bible properly teaches unending suffering for those who go into the Lake of Fire. The authors of the Left Behind books chose to go with the latter doctrine in their depiction of the Lake of Fire.