What is the difference between brown dwarfs and Jupiter?
Jupiter is a planet, brown dwarfs are too big to be planets.
A brown dwarf is an object which is too big to be a planet but too small to be a star.
Brown dwarfs have more than twice the mass of Jupiter. They are smaller than 0.08 solar masses which is the lower mass limit for fusion reactions to start.
The existence of brown dwarfs was only confirmed in the 1995. They have been detected in dust clouds by the infra-red radiation they emit. They are thought numerous, perhaps as many as there are stars.