How is a protostar different from a star?
Imagine "Proto" as Pre-
A protostar is the stage in a star's life before it is hot enough to fuse hydrogen (13 million K). Fusion is the opposite idea to the atomic bombs dropped on Japan. Those bombs split atoms (specifically U-235?), where as in fusion, atoms are fused together. Protostars are simply out hot enough to fuse Hydrogen, and therefore are not main sequence stars like our sun.
Main sequence stars are stars that fuse hydrogen and exhibit a state of "Hydrostatic Equilibrium". Hydro-what? HE (as I'll call it) is where the fusion in a stars core counter acts the force of gravity attempting to compress the star.
Protostars do not exhibit this characteristic, rather they are in a state of collapse until they are compressed to a point where pressure is high enough to have a temperature high enough to fuse Hydrogen.
Protostars do fuse protons to Deuterium (a isotope of helium) but don't worry about this.
Protostars are technically stars, but I think you were asking how they defer from other stars.