The term protostar (or protostellar object) refers to
objects on their way to becoming stars,
and is a common term for some stages in the development of stars.
In a general way, a pre-main-sequence star (PMS) is a star that is shining though not
yet powered by fusion, and the term protostar is generally used
for the stage prior to the PMS stage.
A protostar forms from a contracting molecular cloud
and eventually (on the order of 10 million years)
becomes a T-Tauri star or other type of PMS.
Through these stages, the developing star is heated by the release
of gravitational potential energy as the gas contracts (Kelvin-Helmholtz mechanism),
and eventually reaches conditions to ignite fusion and,
if sufficiently massive, enters the main sequence.
Some related terms: