It's based on colony cost and if you have eccentric orbits on, then the CC of a body might change during its orbit. Plus, as your colony cost reduction tech gets better, bodies might switch from one category to another. But Emu has the gist of it right, it's roughly Earth-like, Mars-like, Galilean moons -like, Venusian, and "the rest" which are never shown on that tally, meaning LG or HG bodies, and uncolonizable bodies.
It is kinda useless, the only time I use it is when I want to see, at a glance of the galaxy map, what systems have "new" easily colonizable bodies after researching a new level of CC reduction tech. Because generally the decision on where to build a new colony is based on either minerals or strategic needs, not the number of possible easy colony sites.