Brown dwarfs in the most simplistic terms is a star remnant. It is usually 10 times Jupiter mass and maybe 3 % mass of the Sun. When the star loses its steam it starts losing its shine and become a white dwarf. What happens in a brown dwarf is that the temperature that a star must need to undergo nuclear fusions, cannot be reached. Because of that the star loses its luminosity. As the star ages its temperature goes down further and further. These stars are also called as "failed stars". So how the star remain. It has something to do with the electron degenracy pressure generated from the inside which balances out the contraction taking place. This is similar to the white dwarf, only white dwarfs have more energy. Also these stars have really high magnetic fields, due to the convective currents at the center of the star
How can one find a brown dwarf?
The popular way is what is called as the litmus test which tests for some chemical signature in the star.
Why is brown dwarfs important?
The main reason for research in brown dwarfs comes from the fact of trying to find hidden matter. It is said that only 10 % of matter has been accounted for in the universe, which makes 90 % unaccounted for. Discovery of brown dwarfs is an important step to trying to find this hidden matter.
Brown Dwarf photos:
Available at the Chandra Observatory site: