Vajradhara (Tib. Dorje Chang) is the ultimate primordial Buddha, or Adi Buddha, according to the Gelug and Kagyu schools of Tibetan Buddhism.
In the evolution of Indian Buddhism, Vajradhara gradually displaced Samantabhadra, who remains the 'Primordial Buddha' in the Nyingma, or "Ancient School." However the two are metaphysically equivalent. Achieving the 'state of vajradhara' is synonymous with complete realisation.
According to the Kagyu lineage, Vajradhara is the primordial Buddha, the dharmakaya buddha. He is depicted as dark blue in color, expressing the quintessence of buddhahood itself and representing the essence of the historical Buddha's realization of enlightenment.
As such Vajradhara is thought to be the supreme essence of all (male) Buddhas (his name means 'The bearer of the thunderbolt'). It is the Tantric form of Sakyamuni which is called Vajradhara.
Tantras are believed to have been originally taught by the Tantric form of Sakyamuni called Vajradhara. He is an expression of Buddhahood itself in both single and yabyum form.