Mahayana is the largest thing in Buddhism and the hardest to point at. It is not a denomination, not a church and not an order. It has no head, no membership and no polity, and there is no body from which one could be expelled. It is a family of texts, a goal, and a set of philosophical schools that spread across the traditions of East Asia and the Himalaya, and most Buddhists alive today belong to it without ever using the word.
What holds it together is the bodhisattva ideal. Where the earlier schools held up the arhat, who ends the defilements and passes out of the round of rebirth, the Mahayana sutras hold up the being who vows to attain full buddhahood for the sake of every other being and who therefore stays. The change is not in the ethics but in the account of what the path is for.
Around that came a distinctive analysis. Nagarjuna, perhaps in the second century, developed the teaching of shunyata, emptiness: that nothing whatever possesses an independent essence, including the very elements into which earlier analysis had broken down the person and the world. The Yogacara school worked instead on the constructive role of consciousness. Later Mahayana added the doctrine of buddha-nature, the claim that the capacity for awakening is not acquired but uncovered.
The sutras present themselves as the word of the Buddha, taught but not previously disclosed. Theravada does not accept them as such. That disagreement is old, it is real, and it is not resolvable by any evidence both parties would accept.