For a classification of all stars, see The H-R Diagram.
Subdwarf stars (classified as VI luminosity class) are a special category of stars. they have a high metalicity (high abundance of elements heavier than helium). These heavier elements act like an insulation layer, not allowing much heat to escape into cosmos. As a result, these stars radiate less heat and live more than Main sequence stars.
Their high metalicity has also another effect. Subdwarfs emit part of their light in ultraviolet spectra, further diminishing the amount of visible light. It should be noted that massive stars behave in the same way if they have a high metalicity. Only small, M - type stars and K - type stars can be subdwarfs.
The vast majority of subdwarfs correspond to dimmer M-Type Stars, K-Type Stars and G-Type Stars. There are even subdwarfs able to sustain hydrogen nuclear fusion that look like Brown Dwarfs. There are, however, a few unusual types of subdwarfs. Their origin is mostly unknown. They are the O-Type Subdwarfs, B-Type Subdwarfs and Heavy Metal Subdwarfs.
Hosted planet[]
The Habitable Zone is smaller around subdwarfs then around main sequence stars. Still, planets can exist and terraforming is possible.