Bipedalism is a form of movement on the ground. Some species have evolved the ability to move on two limbs. Those that do use their rear limbs. Some animals move on two limbs all the time, they are called habitual bipeds. Optional bipeds have an alternative (moving on four legs).
Though quadruped animals use all the four limbs for locomotion but in some mammals and other phylums locomotion takes place only by hind limbs while the forelimbs support to keep the balance of the body.
However there are some optional bipeds who stand on two legs to drive away competitors and predators, to show courtesy etc. But their locomotion takes place by four limbs.