During the formation of solar systems not all stars begin the fusion process turning hydrogen into helium. A star must have a mass of about 8% of our suns mass to be able to begin to shine. If they are smaller than that they are called brown stars but they can still have disks of matter to form planets. If a forming star has over 10 times the mass of Jupiter they can briefly fuse deuterium in helium but then they shut down. This ability to briefly fuse deuterium is what astronomers use to distinguish brown dwarfs from planets.
(The Great Courses – The Life and Death of Stars)