A dwarf planet is a celestial body very similar to a planet in that it orbits around the Sun and has sufficient gravity to assume a hydrostatic (approximately spherical) equilibrium shape but does not have an unobstructed orbit.
An example is Ceres which, located on the asteroid belt, has the path of its orbit full of those little stars.
At the moment three dwarf planets in the solar system are known, they are: Pluto, Eris and Ceres.
Pluto and its satellite
The distinction between the dwarf planets and the other eight planets is based on the inability of the dwarf planets to clear the vicinity of their orbits, that is, to remove small bodies whose orbits cause them to collide, capture or suffer gravitational disturbances.
The concept is combined with a notion of orbital domain measured in terms of the radius of the mass of a planetary candidate with the combined total mass of all other celestial bodies in its vicinity. Dwarf planets are thought to be too small in mass to significantly alter their environment the way a planet would.