Brown dwarfs are objects with masses, effective temperatures and luminosities intermediate between those of very low-mass stars and the most massive giant planets. In the last decade, numerous searches have revealed their ubiquitous presence in star forming regions and stellar clusters, orbiting stars and free-floating in the field. Hundreds of brown dwarfs have been identified via direct imaging techniques. Brown dwarfs appear to be as numerous as stars. Follow-up spectroscopic observations have been crucial for establishing their properties. New spectroscopic classes have been required for these objects. The L and T brown dwarfs form a unique laboratory in which to test substellar atmospheric and evolutionary models. We briefly review the photometric and spectroscopic properties, the multiplicity, mass function and possible formation scenarios of these substellar objects. Old low-mass brown dwarfs are expected to cool down to atmospheric temperatures similar to those of the planets in the Solar System. Their atmospheric properties will guide future planet searches.