I'm not exactly sure how it works in brown dwarfs fusing deuterium, which is only a tiny fraction of the total hydrogen, but stars only fuse elements in the core. When that's used up, the star dies. The more massive the star, the shorter the lifespan on the order of around mass to the power of -3.5. This is a lengthy topic, however, but the Wiki page should give you a decent overview:
http://en.wikipedia.org/wiki/Stellar_evolution