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Neptune
Neptune is the eighth planet of the solar system, and the one located furthest from the Sun. It is a gas giant that is 17 times more massive than the planet Earth, and like other gas giants, it is made mostly of hydrogen and helium. However, the outer gas giants of Neptune and Pluto are different from the inner gas giants of Jupiter and Saturn in that they have large trace amounts of water, methane and ammonia ice. Neptune has 13 known satellites, and is notable for its visible weather patterns. Like Jupiter, the surface of Neptune has a gigantic, perpetual storm that has been dubbed "the Great Dark Spot."
Pluto
Pluto was formerly the ninth planet, but in a 2006 reclassification, it was placed in the Kuiper Belt, and therefore ceased to be a categorical "planet." This reclassification was driven by the discovery of numerous objects beyond Pluto in recent years, and the realization that Pluto has more in common with them than with the other planets. Pluto has one-fifth the mass of Earth's moon, with a diameter of 2,390 km. It is made up mostly of rock and ice.
Pluto's Orbit
When Pluto was considered a planet, it has a highly unusual eliptical orbit. The solar orbits of the eight planets are all roughly circular and on the same orbital plane. Pluto's, on the other hand, orbited on a different and highly eccentric plane, and was shaped like an elipse. The result was that Pluto sometimes moved into a position that placed it closer to the sun than Neptune. Pluto last moved into this closer position in 1979, and then back out again in 1999. Pluto will not move closer to the sun than Neptune until 2227.
Crossing Orbits?
When the solar orbits of the planets and Pluto are viewed as a two-dimensional overhead, it looks like the two orbits intersect. This has given rise to the belief that Pluto "crosses" Neptune's orbit, and that it may be possible for the two celestial bodies to collide. However, they two planets cannot collide anytime soon, and for a variety of reasons.
Reasons Why They Cannot Collide
When viewed in three dimensions, it becomes obvious why the two planets cannot collide. When Pluto is in a position to "cross" Neptune's orbit, it is on a much different orbital plane, far above or below Neptune's position. Therefore, even if Neptune were in the vicinity, Pluto would be passing Neptune from a position far away and not on a collision course. Furthermore, the closest Pluto ever comes to Neptune is about 2 billion km. It is possible that gravitational effects so subtle and far in the future could produce changes in the orbit of Pluto that it could collide with Neptune. However, this would be at least 1 million years into the future, and likely even longer.