The Ares I rocket, pictured in the drawing to the left, will carry the astronaut crews into space. The capsule, or crew exploration vehicle, has three time the volume of the Apollo era capsules. The new Ares capsule will accomodate six astronauts compared to the three aboard the Apollo capsule. Also, the capsule can be reused up to 10 times as the heat shield is released as part of the reentry process. The new capsule will also parachute to land as opposed to the ocean landings during the Apollo era.
The rocket will utilize two stages - a reuseable solid rocket booster derived from the shuttle program and a convetional oxygen/hydrogen  powered second stage.
The rocket motor for the second stage is an evolved version of the Rocketdyne J-2 engine that was used on the Apollo missions. The new engine, dubbed the J-2X, was chosen over the engines used on the space shuttle orbiter.
The new rockets will begin their first testings in 2009, launching from the Kennedy Space Center. Using elements that have already been tested should speed development.
The basic physics of leaving Earth orbit has not changed since the 1960's; right now brute force is the only way we know to escape the Earth's gravitational pull. This means large rocket motors that consume huge amounts of fuel.
Many of the space shuttle's problems will be avoided with the Ares design. Given that the crew capsule is at the top of the rocket, falling debris cannot harm the capsule. Also, if a mishap were to occur, the capsule has a small rocket that would jettison the crew capsule to safety. NASA estimates that this design from the Apollo era is ten times safer than the current shuttle design. 
The exploded diagram on the left shows all the major components of the Ares I rocket. Solid rocket boosters have proven reliable launch vehicles for both manned and unmanned space missions. The second stage with the new J2-X engine, will propel the vehicle into orbit.
The Ares I Rocket
Artist conception of a Ares I launch
The J2-X is unique because it can be shutdown and later restarted. Not only is it used in launch, but it will drive the crew capsule and the lunar lander to the Moon. The J2-X was designed to be used in space. To learn more about the Ares I, please download the following PDF file, from the NASA website: Ares I Factsheet.