A brown dwarf is an object which is made of the same things as stars, but does not have enough mass for hydrogen fusion (the combining of hydrogen atoms into helium atoms). Nuclear fusion is what makes stars glow. Brown dwarfs are not massive enough to do this, so they are not regular stars. On the other hand, they are not regular giant planets, because they do glow. They are thought to be many but few have been found because their absolute magnitude is small.