More info on brown dwarfs.

A brown dwarf is an object that lies in the mass range between a large Jovian planet and the smallest stars. They are considered to be failed stars. The upper limit is well defined at around 80 Jupiter masses. Beyond that, the center of the object is hot enough to ignite nuclear fusion and shine to become a star. There is some debate over the lower limit, but recent discoveries put it at around 10 Jupiter masses. To look at, they would be about the same size as Jupiter but much denser. They would probably glow a dim red as the there is huge amount of heat left over from their formation.

I think the best way to distinguish between a brown dwarf and a Jovian planet is that a planet is formed from the gas and dust surrounding a newborn star, where a Brown dwarf is formed in the same way a star is: directly out of a interstellar nebula. But here is where the debate over the definition of the lower mass limit arises. For example:

It is quite possible to have a planet that weighs say, 10 Jupiter masses that is formed in a solar system and at same time have a similar object formed in an intersteller gas cloud that weighs the same. Which one is a planet and which is a brown dwarf? This is where the debate is ongoing on.