The main purpose of the Constellation program is to design and build the spacecraft for America's next generation of human spaceflight. The program's current projects are the Ares I and Ares V launch vehicles, the Orion crew capsule, the Earth Departure Stage and the Altair lunar lander. Once completed these spacecraft will be able to perform all of America's duties in space such as supplying the ISS, and hopefully landing on the Moon and eventully Mars.
The main goals of the NASA's Constellation program are to keep an American presence in suborbital space, to return to the moon and hopefully establish an outpost, to prepare to explore Mars and possibly get humans there by 2050.
The development of this program was started politically by the Vision for Space Exploration which George Bush proposed in 2004. This Vision has good intentions and should keep NASA busy, though I would imagine, and almost hope, that private companies exceed NASA's unambitious goals.
But to me the Constellation program is interesting and has a good bit of potential so I will continue this series with the next part being on the Orion crew exploration vehicle.
Image source: nasa.gov