A Young Stellar Object (YSO) is a star in the early stages of development, i.e., a Protostar or a Pre-Main-Sequence Star (PMS). They can be hard to see because their normal environment, a Star-Forming Region (SFR) usually suffers from Extinction.
Classes of YSOs:
dlog(λFλ) α = ————————— dlog(λ)
The classes roughly follow the evolution. They also often show Jets, disks, etc.
Herschel Orion Protostar Survey (HOPS)
Spitzer Space Telescope (SST)