Pluto is the furthest of the planets that make up our solar system.  It is very small, about one-fifth of the diameter of Earth, and so far out that it was not discovered until into the 20th century-- and then only because it was blocking the view of stars.  Pluto has one moon, so large that it's almost a double planet.  Its orbit is on an angle to the relatively flat plane of the orbits of the other planets, and so elliptical that it swings inside the orbit of Neptune at times and then swings out.  It's orbit is so huge that it has only gone a fraction of its orbit since we have discovered it.  Pluto reflects light back from the sun quite readily, leading to speculation that it may be ice covered.

More information on Pluto:

http://pds.jpl.nasa.gov/planets/choices/pluto1.htm

http://www.seds.org/nineplanets/nineplanets/pluto.html

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