Who is Lews Therin from Lord of Chaos and what is their importance?

Lews Therin was the Dragon Reborn for the last Age, the Age of Legends. He was responsible for sealing up the Dark One in his prison. The current Dragon Reborn is Rand, and because of this link Lews Therin is somehow a part of Rand mentally. Rand constantly deals with cryptic snippets of Lews Therin's voice. Lews went very mad and evil with magic, and his bloodlust is something Rand must constantly fight against. In earlier books, the implication is that Rand is actually Lews Therin reincarnated.