A brown dwarf (BD) is a star-like object which has insufficient mass to sustain fusion in the manner of main sequence stars ("stable fusion", i.e., of 1H) but with enough to trigger deuterium fusion. This is roughly (and standardized as) 13 to 75 Jupiter masses. They can have an effective temperatures ranging from 250K to 3000K, and any of the spectral classes, M-type star (M dwarf), L-type star (L dwarf), T-type star (T dwarf), or Y-type star (Y dwarf). They cool over the course of their life, so their classification evolves, which presents a mass/age degeneracy: that a young brown dwarf within the lower end of the mass range has an Teff (and spectral class) identical to that of more massive brown dwarfs that are sufficiently older.
The coolest main sequence stars are within the hotter end of the brown-dwarf range (having m classifications), and many planets are warmer than 250K. Consequently, even with the spectral class and absolute magnitude determined, its mass is needed to determine whether the object is a planet, a brown dwarf, or a star. Thus, when candidates (substellar objects of some kind) are found at distant orbits from a star, so mass-determination is possible as well as spectrography from direct imaging, characteristics of such bodies can be collected, which, in turn, assist in classifying similar objects found nearer the stars, e.g., by transits. Modeling such bodies has the extra complication of the possibility of clouds in the atmosphere.
The early-established observables used to identify brown dwarfs were the presence of lithium and/or methane. The first observed brown dwarf was identified as such in 1995. As of 2013, hundreds of brown dwarfs are known.
Brown dwarfs' rotation periods are generally hours, typical of a planet, rather than multiple days, typical of a star. They lack stars' phases of stellar wind, something that reduces angular momentum. Also, they grow smaller as they cool, causing their rotation to increase.