The Orion Service Module will differ from the Apollo SM by using solar panels, rather than fuel cells, for electric power. The Orion SM was to use liquid methane as fuel, with liquid oxygen as oxidizer. The decsion to use liquid methane fuel for the first time, requiring new engine development work, was made because methane might be manufactured from the Martian atmosphere on future Mars landing missions, and because the methane/LOX combination provides higher specific impulse (thrust per pound of propellants) than the hypergolic hydrazine/nitrogen tetroxide combination used on the Space Shuttle OMS engines and the Apollo SM.
However, in 2006 NASA decided to abandon the liquide methane fuel plan in order to reduce Orion's development time. Thus the Orion SM and the LSAM lunar lander ascent stage will use the same reliable hypergolic hydrazine/nitrogen tetroxide fuel and oxidizer as Apollo and the Space Shuttle.
There are to be four versions of the Orion CEV:
Block 1A: for transferring crew and cargo to and from the International Space Station.
Block 1B: a pressurized but unmanned version for transferring cargo to and from the ISS.
Block 2: for lunar landing missions, in conjunction with a Lunar Surface Access Module.
Block 3: Earth reentry vehicle for Mars landing missions, in conjunction with an as yet unspecified Mars Transit Vehicle.