M-class planets, in Star Trek, are those planets whose attributes (climate, gravity, air makeup) make them suitable for habitation by humanoid species such as Terrans and Vulcans.
Other classifications include Demon-class worlds, which are extremely hazardous to humanoids, who usually require environmental suits to survive even for a brief amout of time on them. The U.S.S. Voyager explored a Demon-class world once, on which bio-mimetic fluids took on the forms of all the crew and set out--unaware that they were only copies--to complete Voyager's mission, that is, to get "home" to the Alpha Quadrant and the Federation.