Class M is the classification used in Star Trek to refer to planets suitable for supporting life as we know it. They usually have oxygen/nitrogen atmospheres and are in a temperature range similar to Earth's.
Many people have complained about how many planets in Star Trek are Class M. The reason for this is simply one of budgets. Scenes taking place on a Class M planet can be easily filmed on Earth. Other planets need detailed sets made in studios.
Well known Class M planets in Star Trek are: Earth, Kronos and Bajor.
On the Star Trek series Enterprise, Sub-commander T'Pol suggests that Class-M stands for Minshara-Class, a Vulcan term.
