When Pluto was discovered in 1930, it was thought to be a one of kind anomaly, a small icy planetoid lost in the outer regions of the solar system. One common theory to explain its existence was that Pluto may have been a satellite of Neptune that had gone rogue. For those reasons (small size, possibly an escaped satellite), many scientists at the time thought that Pluto should not be considered a planet, but a majority of astronomers decided otherwise and Pluto became officially the 9th planet.
At the end of the 20th century with the invention of more powerful instruments, astronomers realized that Pluto was absolutely not an 'anomaly', it was discovered that Pluto is actually part of what is called the Kuiper Belt, a large ring of thousands of objects orbitting the outer regions of the solar system. In recent years, several planetoids totally similar to Pluto (though smaller) have been discovered in the Kuiper Belt, starting the debate on whether those new objects should be considered planets or not.
Finally, last year, a new Kuiper Belt object named 2003 UB313 was discovered. This one being significantly bigger than Pluto, the number of planets in the solar system had to be revised, it was impossible to keep Pluto (officially part of the Kuiper Belt) as a planet while not giving the status to a similar bigger object.
The IAU (International Astronomical Union) is currently meeting in Prague and will officialize the new definition of 'planet'. The definition has been made as simple as possible:
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The part of "IAU Resolution 5 for GA-XXVI" that describes the planet definition, states "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."
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According to that new definition, the Solar System will now have 12 planets, the 3 new ones are:
Ceres, an object in the Asteroid Belt between Mars and Jupiter.
Charon, originally considered a satellite of Pluto but now regarded as forming a twin planet with Pluto.
2003 UB313, which is obviously a temporary designation, what is to become the 12th planet will receive an official name probably later this year.
Picture of the new Solar System.
Picture of the 3 new planets.
Full IAU press release.
All scientists agree that many more objects fitting the new definition of planet will be discovered in the Kuiper Belt in coming years, so the Solar System will likely expand to 20 or 30 planets.
It is to be noted that the 'classical' 8 bigger planets are considered a special class of 'major planets', Pluto on the other hand is grouped with the other Kuiper Belt planets. So to make it simple, the general public can consider that there are 8 [major] planets (that won't change), and 4 minor planets which will be joined by many others in the coming years as the Kuiper Belt is being explored.