Pluto, the former planet of our solar system, is now found to have 5 moons. This new moon just followed the recent discovery of its 4th moon, P4, last year. The Pluto-system now consists of the Pluto & Charon, Hydra and Nix, P4, and now P5 as of July 2012.
Astronomers using NASA's Hubble Space Telescope were able to observe the moon orbiting Pluto from images taken with Hubble's Wide Field Camera 3. P5 (not an official name yet, likewise with P4) is estimated to be irregular in shape ranging from 6-15 miles across. It's orbit around Pluto has a diameter of 58,000 miles.
Although it is now found to have more moons, Pluto cannot yet retain its planet classification. The number of small solar system bodies around Pluto's orbit (which may be the reason behind the growing number of moons) shows that it still violates the 3rd condition of a planet.
According to the IAU (International Astronomical Union) a planet is a body:
a) in orbit around the Sun,
b) has sufficient mass to achieve a hydrostatic equilibrium shape (spherical shape); and
c) has cleared it's neighborhood around its orbit.
The presence of the Kuiper belt objects removes Pluto from the planet category and down to the dwarf-planet category (defined by satisfying condition a and b) with it's partner Charon. All other bodies that only match condition a are classified further into SSSBs or Small Solar System Bodies (asteroids, meteoroids, etc.).
We will get to understand Pluto further when the New Horizon's spacecraft will do a flyby on 2015.