When Pluto was found back 1930 as the ninth planet, people had always argued whether it was a planet or not.
In the mid 00's, Eris was discovered. It was larger than Pluto but smaller than our moon. This meant a new actual planet was discovered which they didn't seem to want or something like that, and the likely hood of many more planets like it floating in our Solar System so the Dwarf Planet Catagory was formed and Eris and Pluto were now classified as Dwarf Planets. Ceres which was discovered sometime in the 1800's was also classified as one.
So basically Pluto is no longer the ninth planet of the Solar System, that's basically it in a nut shell