A red supergiant is the biggest type of star and one of the brightest. They're cooler, and therefore dimmer, than blue supergiants. Because they're so large, they tend to burn their fuel quickly in order to support their own weight and so they burn out in only a few million years. For this reason, they're very rare.
They have large masses and low densities and are often around about 100 times the diameter of the sun.
Supergiants are pretty much all gas with very little solid matter therefore, despite their enourmous size, they tend to be very low mass.
These stars were formerly high-mass main sequence stars.