Bipedalism

Bipedalism is a form of movement on the ground. Some tetrapods 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" can move on two or four legs.
Early quadruped animals used all four limbs for locomotion but many later ones are bipedal. The earliest dinosaurs were bipedal but millions of years later some reverted and became quadrupeds. Birds are descendants of bipedal dinosaurs, and are bipeds themselves. Their forelimbs have become wings. 
Some optional bipeds stand on two legs to drive away competitors and predators, to see farther, or as body language. Their locomotion is on four limbs.