Class-M planets generally have a diameter of 10000-15000km and normal cores, their atmospheres are primarily composed of nitrogen and oxygen (making them humanoid-breatheable), they have humanoid-tolerable climates (in some way--Vulcan tends to get extremely hot, but is easily tolerable by its own inhabitants), and abundant liquid water (up to 97% of the surface area--any more under the same conditions, and it's considered Class-N).
In Enterprise, which technically is not canon to the rest of the Star Trek Universe, it was revealed that the letter "M" stands for the Vulcan word "Menshara".
Well-known Menshara-class planets include Sol-3 (better known as "Terra" or perhaps "Earth"), Vulcan, Kronos, and most of the places the crews of the various Star Ships Enterprise have been to.