So, what exactly is "Thermonuclear Fusion"?
As gravity pushes the cloud matter together, hydrogen atoms move very fast. Hydrogen atoms start to fuse together forming helium atoms and providing the cloud with energy and heat.
Thus, what makes a star a special object is that a star is self-luminous. In other words, a star produces its energy by its self through nuclear fusion process.
If the protostar did not have enough dust and gas accreted to achieve the critical temperature to ignite fusion, then it is called a brown dwarf.