Answer:
A protostar looks like a star but its core is not yet hot enough for fusion to take place. The luminosity comes exclusively from the heating of the protostar as it contracts. Protostars are usually surrounded by dust, which blocks the light that they emit, so they are difficult to observe in the visible spectrum.
Step-by-step explanation:
A protostar is not considered a star until it gets its energy from nuclear fusion instead of gravitational contraction. Gravitational contraction is the process of a cloud's gravity overcoming its internal pressure and causing a collapse.
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.
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