Mahayana, which means "Great Vehicle" in Sanskrit, is one of the two major schools of Buddhism. It emerged as a separate school from Theravada, the other major school, during the 1st century BCE. The origins of Mahayana probably go back to at least the 4th century BCE, however, and possibly to the very beginnings of Buddhism.
The major doctrinal point that distinguishes Mahayana from Theravada is that of shunyata, or "emptiness." Shunyata is a deepening of the doctrine of anatman, or anatta, which is one of the foundational teachings of all Buddhism. According to this doctrine, there is no "self" in the sense of a permanent, integral, autonomous being within an individual existence.
The Theravada school interprets anatman to mean that an individual's ego or personality is a fetter and delusion. Once freed of this delusion, the individual may enjoy the bliss of Nirvana.
Mahayana teaches that beings and phenomena have no intrinsic existence of their own and take identity only in relation to other beings and phenomena. Shunyata also is an absolute reality that is all things and beings, unmanifested.
The ideal of Mahayana practice is the bodhisattva, "enlightenment being," who works for the enlightenment of all beings.
Some people speak of three schools of Buddhism, and call them Hinayana ("small vehicle"), Mahayana ("great vehicle"), and Vajrayana ("diamond vehicle"). However, Hinayana often is considered merely a pejorative term for Theravada, and Vajrayana is best understood as an extension of Mahayana, not a separate "vehicle."