An M dwarf is a small M-type star. The term red dwarf includes M dwarfs, but sometimes also includes small K-type stars (K dwarfs). Early M dwarfs (at the hotter end of the scale) are now considered likely places to observe extra-solar planets within a habitable zone, one reason being M dwarfs are very common. But given their evolution, such planets are more likely to have experienced a longer period of stellar activity, giving it time to remove the planet's atmosphere.
Examples include: AD Leonis, Barnard's Star, Lacaille 9352, Lalande 21185, Ross 154, Ross 248, Scholz's Star, and Wolf 359.