What is a brown dwarf?
A brown dwarf is a substellar object unable to sustain long-term hydrogen fusion like an ordinary star. Brown dwarfs occupy a mass range between giant planets and the smallest stars. Their name does not mean all would look brown to the eye.
Differences from stars and planets
Gravitational contraction supplies heat during formation, and temporary deuterium fusion can occur over some mass ranges. This does not make a long-lived main-sequence star. Boundaries depend partly on composition and definitions. Distinguishing a planet from a brown dwarf can involve formation history and observational context as well as mass. Its spectrum changes as it cools.
How are they detected?
Infrared observations are important because visible emission is weak. Spectra reveal temperature, atmospheric chemistry, and clouds, while binaries can help constrain mass. Faintness alone is insufficient for classification because distance also dims objects. Brown dwarfs illuminate the low-mass end of star formation and giant-planet atmospheres. They differ from hypothetical black dwarfs, the far-future cooled remnants of white dwarfs.