A Young Stellar Object (YSO) is a star in the early stages of development, i.e., a Protostar or a Pre-Main-Sequence Star. They can be hard to see because their normal environment, a Star-Forming Region usually suffers from Extinction, so, for example, they are often not recorded by Gaia. Fortunately, many have significant Magnetic Fields and are Radio Sources (Radio YSOs).
Classes of YSOs:
Where α is its "wavelength" Spectral Index:
dlog(λFλ) α = ————————— dlog(λ)
Where λ is the Wavelength and Fλ is the Flux density.
The classes roughly follow the evolution. They also often show Jets, disks, etc.