What with all the talk of exoplanets recently, it's easy to forget about brown dwarfs. But that's not terribly fair! These are objects much more massive than planets, but which don't quite have enough mass to fuse hydrogen into helium in their cores — an object that can do that, by definition, is a star.
Some people call them failed stars, which I don't like. Maybe they're just overachieving planets!
The upper limit to the mass of a brown dwarf is about 80 times that of Jupiter (or, if you prefer, about 0.08 times the mass of the Sun). That's not hard-and-fast; under some conditions brown dwarfs can be more massive than that, but as a rough number it works.
The lower limit is harder to define. Once you get up to about 13 times Jupiter's mass, an object can fuse deuterium (an isotope of hydrogen) in its core, though this doesn't last long. Still, it's a decent place to make a cutoff, so we say that an object with a mass between 13 and 80 Jupiters is a brown dwarf.
The first true brown dwarf, Teide 1, was discovered in 1995, and the second, Gliese 229b, was ...