"Planet" means "wanderer", because these object moved across the sky relative to the "fixed stars". Originally, heavenly bodies were classified as stars, planets, comets, meteors. There were exactly 7 "planets" in the original definition: Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, after which, in many civilizations quite independently, the days of the week were named. (Ever wonder why there are 7 days in a week in most cultures? Now you know.)

Then Copernicus, Kepler and the boys came along with the "Heliocentric System". This upset many people when it asserted that the Earth, which is never seen above (except by losers in bar fights), is also a planet, and the Moon is not. It redefined planet as any object traveling around the Sun. (Incidently, Kepler was slightly wrong about the Solar System. The planets do NOT orbit the Sun. Rather, the planets and the Sun orbit the common center of mass of the Solar System which is sometimes within the body of the Sun, other times not, although never very far away. Tycho Brahe's observations were probablly accurate enough to show this error.)

So the new definition stood for hundreds of years. Uranus, Neptune were added to the list of planets as they were discovered. Meanwhile, the asteroids (planetoids) were discovered. They were so much smaller than the known 8 planets, that they were given a different classification all their own. Comets were defined and meteors (the glowing manifestations) were found to exist only within the atmosphere, although they were produced by meteoroids from outside the atmosphere. Things became nice and orderly, and everything fit in its proper taxonomic niche.

Then, in 1930, Pluto was discovered. Because it orbited the Sun in very roughly the same plane as the other planets, and seemed to be larger than Ceres (the largest asteroid or planetoid), it was declared to be a planet. Time went by, astronomers juggled their definitions in response to discoveries of more and more objects in the Solar System. The demotion of Pluto was just another step in this juggling. Pluto is still exactly what it was 50 years ago, or 50 million years ago, for that matter.

What I don't understand is why people are so upset about this. For thousands of years, the classification of "planet" included the Sun and the Moon. Then they were booted out. Uranus, Neptune, and finally Pluto were added. Now Pluto gets the boot. Big deal!