Every major religion has several different belief systems (sects) that have the same general core belief, but have a significant enough different to have be separated from each other.
What are the major (3-10) sects of Buddhism? How are they different from each other?
Mahayana is not a sect at all; it is a vehicle.
Major traditions in Buddhism are usually country or region related. We can distinguish three major branches of Buddhism in the World by what they keep as canon.
Some countries mix several aforementioned branches.
The three major sects would be Hīnayāna - the small vehicle/path; Mahāyāna - the great vehicle/path and Vajrayāna - the diamond vehicle/path. For differences compare this thread: What are the main differences between the Theravada, Mahayana and Vajrayana branches of Buddhism?
