Simply explained as a "Rotating sphere of hot ionized gas"
Spectral Type M: Occupying the lowest range (lowest in surface temperature) ; Color: "red" ; App. Surface temperature: under 3,500 K ; Average Mass (The Sun= 1): 0.3 ; Example: Betelgeuse
Spectral Type K: Color: Orange to Red ; App. Surface temperature: 3,500- 5,000 K ; Average Mass (The Sun= 1): 0.8 ; Example: Arcturus
Spectral Type G: Color: White to Yellow ; App. Surface temperature: 5,00- 6,000 K ; Average Mass (The Sun= 1): 1.1 ; Example: Sun
Spectral Type F: Color: Blue to White ; App. Surface temperature: 6,000- 7,500 K ; Average Mass (The Sun= 1): 1.7 ; Example: Canopus
Spectral Type A: Color: Blue ; App. Surface temperature: 7,500- 11,000 ; Average Mass: 3.2 ; Example: Sirius
Spectral Type B: Color: Blue ; App. Surface temperature: 11,000- 25,000 ; Average Mass: 18 ; Example: Rigel
Spectral Type O: Color: Blue ; App. Surface temperature: Over 25,000 ; Average Mass: 60 ; Example: 10_Lacertae
All Main- Sequence Stars hotter than the Sun are more luminous and larger than our Sun
But that is only true for Main- Sequence Stars.
The other 10% of stars that are not Main- Sequence can be: Giants, Super Giants, and White Dwarfs.
Main Sequence Stars: are all basically undergoing Hydrogen Fusion
Giants: bigger and more luminous than Main Sequence stars, have left their Hydrogen Fusion state and are depleted, they will keep at fusion because of storage in their cores, but at a time of 10 to the power of 12, they will later become hydrogen white dwarfs.
Super Giants: "Supergiants can have masses from 10 to 70 solar masses and brightness from 30,000 up to hundreds of thousands times the solar luminosity. They vary greatly in radius, usually from 30 to 500, or even in excess of 1,000 solar radii. The Stefan-Boltzmann law dictates that the relatively cool surfaces of red supergiants radiate much less energy per unit area than those of blue supergiants; thus, for a given luminosity red supergiants are larger than their blue counterparts. "
White Dwarfs: Very High density compared to their size (example: mass of the Sun with size of Earth) They are the remnants of a dying star, when that star has depleted all of its resources and collapses in itself it (these are the typical ones, since they are created by stars not big enough to produce a neutron star.) They stop producing heat, but start losing it, they gradually become cooler and eventually becomes a Black Dwarf -> [These are not known to exist since it takes more time to become a Black Dwarf than what the universe took to this date]
 - Wikipedia search Super Giants: SuperGiants
All other information was summarized by me, to learn even more, click on the links. :)))