The key difference between the otherwise identical planet and dwarf planet is the fact that dwarfs share their orbit space with other objects with similar sizes. In the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant. This means that there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence. The current definition of a planet adopted by the International Astronomical Union only includes those bodies which have "cleared the neighbourhood of its orbit." A large body which meets the other criteria for a planet but has not cleared its neighbourhood is classified as a dwarf planet. In Pluto's case, it shares its orbital neighbourhood with a distinct population of objects called the Kuiper belt (Pluto is the largest of these objects).
NB: Astronomers and other experts are debating the definition of a dwarf planet and whether or not Pluto should be classified as one.
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