Pluto is the farthest planet from the Sun (usually) and by far the smallest. Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).

The pictures above, are the most detailed pictures, astronomers have been able to take, with the Hubble telescope.

According to Roman legends, Pluto (Greek: Hades) is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it's so far from the Sun that it is in unending darkness.

Here is a picture of Pluto's ancient symbol

Pluto was discovered in 1930 by a fortunate accident. Calculations, which later turned out to be in error, had expected a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey, which turned up Pluto anyway.
Pluto is the only planet that has NOT been visited by a spacecraft (in our solar system).
Pluto has a satellite, Charon. With some good luck, Charon was discovered (in 1978) just before Pluto's orbit, went between Neptune, and the sun.
Pluto's orbit is very unusual. At times it is closer to the Sun than Neptune (as it was from January 1979 to February 11 1999). Pluto rotates in the opposite direction from most of the other planets. You may be wondering why Pluto doesn't crash with Neptune, since their orbits are so close. This is because their orbits are in control. Pluto orbits the Sun twice for every 3 orbits of Neptune. As a result, Pluto and Neptune are never very close to each other. Here is a diagram on the planets orbit: