Some scholars call Vajrayana (thunderbolt vehicle) a separate branch of Buddhism (rather than Mahayana) because of its distinctive practices. Tantra is central to this kind of Buddhism. It is best known as the main form of Buddhism in Tibet.
In tantric traditions, unlike most other Buddhist traditions, the practitioner aims to realize enlightenment within this lifetime.
This immediate enlightenment is attained through the practice of esoteric (mystical) practices that focus on the body as the site for enlightenment.
In this sense, tantra shares the Mahayana idea that nirvana is ultimately the same thing as samsara. Thus one can attain enlightenment within this world.
Since all beings actually have inner Buddha-Nature, enlightenment can be brought about in them by whatever means works (such as tantra).