While no designs have been finalized yet, the Altair lander is expected to have two stages, one to house the necessities that a crew will need while on the moon, and another which will propel them to orbit to rendezvous with the Orion Crew Capsule. The Altair lander is expected to hold four people, and to supply and house them for one full week on the surface of the moon.
Unlike the Apollo spacecrafts, in which the Command Module, Service Module, and Lunar Module were all launched as a single spacecraft on a single rocket, the Altair lander and Orion crew capsules are each large enough to require their own separate launches. The completed spacecraft will then be assembled in orbit before continuing on to the destination.
For a trip to the moon, it is sufficient to use liquid oxygen and hydrogen as fuel, but for a trip to Mars, more powerful fuels, like liquid methane, will be necessary. For now, it is not even clear if the Altair lander will be used for trips to Mars, or if another design will be necessary for such a spaceflight.