These straddle the domain between stars and giant planets. Brown Dwarfs are thought to form in much the same way as stars, by the gravitational collapse of clumps of cold gas and dust in dense molecular clouds. For reasons not yet entirely clear, some clumps end up with masses less than about 7.5% of that of our Sun, or 75 times the mass of planet Jupiter. Below that boundary, there is not enough pressure in the core to initiate nuclear hydrogen fusion - the source of power for ordinary stars like the Sun. Except for a brief early phase when some deuterium is burned, these low-mass objects simply continue to cool and fade slowly away while releasing the heat left over from their birth.
See also: Star.