As of 2014, Pluto isn't considered a planet because of its size and location in space. In 2003, when an astronomer found an object in space that was beyond Pluto and also larger than Pluto, it caused scientists to reconsider what constitutes a planet.
The object that caused this change is called Eris. Like Eris, Pluto is now considered to be a dwarf planet. Pluto is also classified as a plutoid, which means that it's a dwarf planet that exists beyond the planet Neptune in space. The three known plutoids are Pluto, Eris and Makemake.
The three qualifications for an object to be considered a planet are that the object must orbit around the sun, it must be a sphere shape due to its own gravitational force and it must have cleared the neighborhood around its orbit. Because Pluto is only slightly larger than other objects in its orbit, it doesn't meet the third criteria.
Grouping similar objects together helps scientists and astronomers to better understand the universe and how it relates to the Earth. The definition of the term "planet" changes as astronomers learn more. Astronomers use telescopes to learn about new objects in space, and their classification is always subject to change as new knowledge is gained.