Protostars are the beginning of a star. The protostar stage is largely what was described on the previous page. Atoms are continuing to collapse to the center of the gas, which puts tremendous pressure on the center of the cloud. In order for the protostar to become a real star, the pressure and temperatures need to become great enough to initiate nuclear fusion.

If the cloud mass doesn’t get hot enough to form a star, then it becomes a brown dwarf. That is the last stage for these kinds of protostars, because they can never become stars.

If nuclear fusion occurs, then a star is created. What kind of star is created depends on a few factors that will be discussed next.

Source
http://aspire.cosmic-ray.org/labs/star_life/starlife_sequence.html (fair use)