Historically the definition of the word 'planet' was uncomplicated: one of a small number of bodies orbiting the Sun. As more and more planet-like bodies were discovered, however, it became clear that by that simple definition the Solar System might in principle to have hundreds or even thousands of planets. In particular Pluto, long considered the Solar System's ninth planet, was found to be just one of a large number of similar objects.
In 2006 a new classification scheme was adopted for bodies within the Solar System, dividing its more significant bodies into two categories: 'planet' for the eight traditional planets from Mercury to Neptune, and 'dwarf planet' for the numerous planet-like lesser bodies, including Pluto.
Like any planet, a dwarf planet is defined as a body that orbits the Sun, and that has sufficient gravity to form itself into a shape close to spherical. The technical difference is that one of the major planets has achieved 'orbital dominance' (that is, it has substantially cleared its orbit of other bodies), while a dwarf planet has not (in most cases, dwarf planets are found in belts of material - specifically the Asteroid Belt or the Kuiper Belt). A further specific requirement is that the body is not a satellite of another planet, to distinguish dwarf planets from moons.
At present there are just five officially recognised dwarf planets, though it is anticipated that the list will expand in the future. Indeed, there are a number of candidate bodies considered likely to achieve dwarf planet status in the near future. Most of these dwarf planets or close candidates belong to the Kuiper Belt beyond the orbit of Neptune, though one, Ceres, is found in the Asteroid Belt far closer to the Sun, while some others have more distant or eccentric orbits.
The total number of dwarf planets in the Solar System is not known with any certainty, because these bodies tend to be immensely distant, making their characteristics difficult to establish reliably. As investigations continue and techniques improve, the ranks of these established dwarf planets can be expected to swell in number.
* This list is does not attempt to list all the prospective dwarf planets (such a list could easily run to hundreds of entries). Bodies listed as 'prospective' are those among the whose parameters place them among the strongest potential candidates.