I emailed the IAU for clarification -- I have to keep on top of this stuff now because my 3.5-yr old nephew is becoming interested in the solar system -- and here's what they told me:
There 8 planets and 3 dwarf planets (Pluto, Ceres and Eris). 2 of the dwarf planets (Pluto and Eris) belong to a sub-category called plutoids.
I can't really understand these without some numbers, so here you go:
Ceres: diameter = 950 km, mass = 1/3rd of the Mars-Jupiter asteroid belt = 0.0002 Earths
Pluto: diameter = 2390 km, mass = 0.0021 Earths, avg. distance to Sun = 39.5 AU
Eris: diameter = 2600 km (greater than Pluto!) = 0.0027 Earths, avg. distance to Sun = 67.7 AU = 1.7 x Pluto
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