Pluto was the first object found at it’s distance from the sun and was declared a planet at the time of its discovery. Eventually, more objects of similar size were found orbiting at roughly the same distance which were not classified as planets. Basically, there had never actually been a formal definition of what made an orbiting object a “planet” in the first place. In 2006 the term was defined, according to size, and Pluto didn’t make the cut (as a reference, Pluto is less than half the size of the Moon). It is now classified as a “dwarf planet,” though it is the largest one in our solar system.
Depending on where that lower size limit to qualify as a planet had been determined, we might have actually added a tenth planet in ‘06 rather than subtracting the ninth; Eris, which is somewhat smaller in size than Pluto, but has more mass.
Anyone who can correct any bits I got wrong; feel free.
haha I was replying very enthusiastically but you were much faster.

I promise I'll leave this thread afterwards, but this burning question was burning me to answer it as I love this stuff. ^^ So here's some additional info I had written. :p
The size is not exactly in the definition of a planet
anymore (before 2006, some astronomers used the size as the official criterium but there was no consensus). Though among other things, it is what made astronomers question its legitimacy as a planet in comparison to other dwarf planets like Eris, and it's pretty much the reason why it doesn't fulfill the current criteria (or its
mass rather than its size to be more accurate, which is smaller than that of Eris).
The official definition for a planet now includes three criteria :
(1) orbiting around the Sun ;
(2) a round shape (vs. asteroids, etc.) ;
(3) during its formation, it has cleared the neighbourhood around its orbit (no nearby objects except its satellites), making it gravitationally dominant.
Pluto and other dwarf planets fulfill the first two, but not the third one. Pluto's orbit is affected by the gravity of Neptune and even its moon, Charon. Not good enough for a planet. :p
If we wanted to include Pluto and Eris (so remove the third criterium), we would have to include over a hundred other spherical objects in the Solar System. The debate is neverending... Defining things in science can be such a shitshow, as nature doesn't "classify", only people do. :p