Pluto is infact a planet. But this planet, however, is unusual. It's a dwarf planet. It's orbit is extremely elongated and odd. One effect of it's elongated orbit is being the eighth planet. As it orbits down the long side of the orbit, it bypasses Neptune and interferes with Neptune's orbit.
Pluto then goes inside of Neptune's orbit and becomes the eighth planet.
At the top of the picture, you can see how Pluto's orbit interferes into Neptune's orbit.

Pluto takes 248 years to orbit the Sun.