Brown dwarfs are celestial objects that are larger than planets but smaller than stars. They have a mass between about 13 and 80 times that of Jupiter, which is not enough to sustain hydrogen fusion like true stars. Instead, they generate energy through the fusion of deuterium and lithium, making them unique in the universe. These objects are often referred to as "failed stars" because they do not reach the necessary conditions to ignite and shine like stars such as our Sun. Brown dwarfs can be found in various regions of space, often in clusters, and are studied to understand the formation of stars and planets.