An M dwarf is a small M-type star. The term red dwarf definitely includes M dwarfs, but sometime that term is also meant to include some or all K-type stars (K dwarfs). Early M dwarfs (at the hotter ones) are now considered likely places to observe extra-solar planets within a habitable zone. An estimate is about a quarter would have such a planet, but another reason is that M dwarfs are very common, comprising about three quarters of all main sequence stars. 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, thus less likely to be habitable. M dwarf examples include AD Leonis, Barnard's Star, Lacaille 9352, Lalande 21185, Ross 154, Ross 248, Scholz's Star, and Wolf 359.