The name 'planets' is derived from the Greek meaning 'wanderers' and was originally applied to the the seven bright bodies that could easily be seen to change their positions in the sky.
Two of these were not planets at all but our Sun and our moon.
Planets are generally divided into two groups;
Mercury, Venus, Earth and Mars are comparable in size and are known as 'terrestrial' planets.
Jupiter, Saturn, Neptune and Uranus, all much larger in size, though having a lower density, are known as the 'major' planets.
Pluto should, by virtue of of its position in the system, be a 'major' planet and by its size, a 'terrestrial' planet.
Because it does not obey Bode's Law, Pluto remains outside of these norms.
Pluto has been declared to not be a planet because its orbit often enters that of Neptune and the scientists claim that only the largest body within an orbit can be a planet.