For this answer, refers to the lecture on the Major Branches of
Buddhism (#7): compare notes from the first slide on Theravada
Buddhism and the first slide on Mahayana Buddhism. For example:
Theravada treats nirvana as something other than samsara, while
Mahayana treats nirvana as the same as samsara. What's this mean? If
nirvana is the same as samsara, then going from the path of striving
after pleasures and avoiding pains to the path of enlightenment is not
to go from one kind of reality to another -- rather, it is to change
one's attitude toward reality. If nirvana is different than samsara,
then one enters a different reality upon attaining nirvana.