Simply put, the International Astronomers Union says a planet:
1--is round and
2--revolves around a star.
So old friends Ceres and Charon, and new friend "Xena," are all planets, according to the new rules. Ceres is in the asteroid belt, Charon is a twin to Pluto, and "Xena" is a nickname for 2003 UB313, larger than Pluto, but much farther away from the sun.
Many argue that the agreed upon definition will make for many more planets. I'm thrilled; we need a firm definition, even if it makes for new planets.
The IAU suggests that we separate the planets into our usual, classical planets and "plutons" will be the descriptor for the others. Beginning, of course, with Pluto, always a hotly-contested planet-contender, plutons are all quite tiny in comparison with the classical planets. As far as I'm concerned, that's that.
Don't you just love picking up on the new terminology?