Apparently there is a new definition of a planet in our solar system, which states that a planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects around its orbit. Under this new definition, Pluto and the other trans-Neptunian objects do not qualify as planets.
There should really be some kind of warning when people change things that have been known for so long so other people can try and keep up, or at least sort of catch up.