A Habitable planet is a Planet where temperatures are right so liquid water can exist. That means the planet is not so hot that all water becomes vapour as round Venus, the planet cannot be so cold that all water is permanently frozen as ice as with Dwarf planet Pluto.

Exotic life that is not based on carbon may theoretically exist on planets that do not have liquid water but most scientists believe carbon life is more likely.

The area round the Sun or any other star where liquid water can exist is called the habitable zone though planetary habitability depends on other factors as well as the distance from the star the planet orbits. Polanets or moons outside the theoretical habitable zone may be habitable if volcanism heat them up (Astronomers suspect that Europa may be habitable through volcanism) or the Greenhouse effect keeps them warm.

Gliese 581 d probably doesn't have sufficient volcanism to heat it up [1] but the greenhouse effect may or may not compensate.