Our Earth is one among many heavenly bodies rotating around a star, the Sun, which gives us light and heat. They all form what we call our solar system. The Earth is a planet. I'm sure you knew that... There are in fact 9 major planets around the sun: Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Only the first five of them (not mentioning the Earth) have been known for thousands of years by many civilizations, from Babylonians to Polynesians: they are the only ones a naked eye can see. The word itself, planet, comes from the ancient Greek word planets, meaning wanderers. While stars look the same night after night, slowly moving along the year and coming back in the sky at the same place at the same time after a year, planets appear to wander in the sky amongst the stars. They follow a path that can be explained only when one realizes that they are orbiting the Sun on elliptic orbits (an explanation that came only in the XVIth and XVIIth century with Copernic and Kepler).
Uranus is just too dim to be seen by a naked eye. It has been discovered at the end of the XVIIIth century by Herschel with a small telescope and recognized as a planet through its motion among the neighbor stars. Neptune is far too faint to be seen without a telescope. Its discovery in 1846 is the result of mathematical computations on the motion of Uranus whose irregularities were attributed to a still to be discovered planet.
Its location was predicted well enough to insure its detection in a relatively easy way. The whole story has been considered as the triumph of Celestial Mechanics, a branch of astronomy that deals with the study of the motion of heavenly bodies.
Pluto is the last one of the major planet. It's definitely different from the others. Discovered in 1930, it looks like a dim star in a normal telescopic image due its relatively small size (2250 km in diameter, 18% of the Earth' size) and its large distance from the Sun, 40 times further than the Earth. It is orbiting in a plane that is far away from the nearly common plane in which the other planets are orbiting (know as the ecliptic). Moreover, the astronomers have discovered that there are many other bodies orbiting the Sun in the same area as Pluto. Some are closer, some are further away. They are called (among many other names) TNOs, or Trans-Neptunian Objects. Mauna Kea played a very important role in this matter as the first TNO has been discovered with the University of Hawaii 2.2-m telescope in 1992. Since then, more than 200 TNOs have been discovered, with more than half of them from Mauna Kea, mainly with the Canada France Hawaii Telescope in the past year. They are considered as remnants from the formation of the solar system some 4.5 billion years ago, and also as a reservoir for short period comets.
Well, if Pluto is a member of a very large family of small bodies, why should it be considered as a planet? Good question! In fact, astronomers thought about making Pluto an ordinary member of the TNO family. Perhaps have you read or heard about that in the news a year ago. Our solar system would have only eight major planets instead of nine... A lot of strong reactions were sent to the branch of the International Astronomical Union dealing with the classification of the solar system bodies. How could a planet be degraded from being major to becoming minor? From Ni'ihau to Hawaii, there are eight islands. But what about Molokini? Here we get our ninth island: as many islands as planets in the solar system. But if we add Molokini to the list, what should we do with Mokupuku and Paokalani, these rocks covered by vegetation off the shore of our Island northeast coast, or with Coconut Island in the Hilo bay? See... everything depends on where we set the limit between major or minor. No need to argue for long if Pluto should be considered as a planet or not. It's good enough to know it's there and what it is. As good as to know the nice crescent of Molokini...