As explained in the ISDB article on stellar evolution, many -- perhaps most -- collapsing protostars simply do not have enough mass to raise their core pressures and temperatures to the point where they will ignite and sustain thermonuclear reactions. A protostar with a mass less than about 75 times the mass of Jupiter (about eight percent the mass of the sun) will, at most, have an initial "belch" of nuclear activity in its core, which will promptly snuff itself out by pushing the outer layers of the protostar outward and rarefying the core. Such a "failed star" is called a brown dwarf and is probably one of the most plentiful, if hard-to-detect, objects in the galaxy.
Young brown dwarfs give off a very dim glow due to their continued gravitational contraction, and this is currently the only way we have of spotting them. At the time I first put the Internet Stellar Database together, many suspected brown dwarfs had been detected indirectly by gravitational tugs on "real" stars they were orbiting, but only one brown dwarf -- Gliese 229B -- had actually been imaged. At the time of this writing, though, several other brown dwarfs have been seen.
The discovery of brown dwarfs as actual (if weak) radiation sources led astronomers to create two new spectral classes:
Adam Burgasser has a webpage devoted to brown dwarfs -- particularly spectral class T brown dwarfs -- at http://www.its.caltech.edu/~diver/homepage/research/tdwarf.html.