From 1930 to 2006, Pluto was considered to be the ninth planet of our solar system. In 2006 it was reclassified as a dwarf planet, though many still consider it the ninth planet for historical reasons. One reason that it lost its planetary status is because it is so small. It has less than one percent of Earth's mass. There are numerous other bodies in that part of the solar system (called the Kuiper belt) with similar mass so the decision was made to make the classification of dwarf planet.
Another reason that Pluto lost its planetary status is because its largest moon, Charon, is not that much smaller than Pluto itself. As a result, many consider the Pluto-Charon system to be binary system rather than a classic planet-moon type system, though it is not technically classified as such.