A toilet in space is hampered by the fact that there is no gravity to hold a toilet bowl full of water in place, or pull human wastes down when you flush.
Hence toilets in space rely on using air vacuums to properly transport wastes around the space craft.
For sitting, the toilet on the space shuttle has straps and bars that hold the astronaut in place. The astronaut is required to ensure that a tight seal exists so that the air pump works properly.
There is also a hose which can be utilized as a urinal by men and women, and this also uses flowing air instead of water to move waste through the system.