The sun is a star--a yellow dwarf star, if you wanna get technical. Stars have a limited lifespan. Ours is about 10 billion years. It's used up about 4.3 billion of those years already. After that, it'll start collapsing and expand into a red giant. That's the point when it takes out Earth and a few of the other planets while it's at it.

Long way off, but most people figure we should get started early. Better not let it catch us by surprise.