Astronomers at IAU Congress in Prague reached agreement that the planets and planet like-bodies in the Solar System (and in other solar systems) will be categorized into three groups, meaning that Earth is still a planet, but Pluto is now a “Dwarf planet.” Here is what they decided.
1. The classic eight planets of the Solar System will constitute the planets. A planet, as defined at the Congress is “a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a … nearly round shape, and has cleared the neighborhood around its orbit.”

Image: Planets of the Solar System

2. Dwarf planets are almost planets in the sense that they meet the definition of planet except for the orbit. For example, Pluto at times is within the orbit Neptune, so disqualifies as a real planet. Thus, the Solar System right now has four Dwarf Planets which include Pluto, and its partner Charon, then Ceres and the recently discovered dwarf planet UB313 (Nicknamed Xena).

Image: Examples of Dwarf Planets, Pluto and Charan

3. Asteroids and comets (as shown here) are classified into the third category of small Solar System bodies or stuff.

This was an interesting chapter in the history of astronomy, and the process used to arrive at a new classification of stuff in the Solar System. This post, and the others at this site, along with the articles that I’ve linked could be a useful resource for an inquiry in a science classroom about the nature of science.