Lord Krsna appears in every millennium, in various incarnations, to annihilate the demons, protect the devotees and reestablish the principles of religion. From upper left-hand corner: Lord Matsya, the fish incarnation; Lord Kurma, the tortoise incarnation; Lord Varaha, the boar incarnation; Lord Nrsimhadeva, the man-lion incarnation; Lord Vamana, the dwarf-brahmana incarnation; Lord Parasurama, the warrior incarnation; Lord Ramacandra; Lord Krishna and Lord Balarama; Lord Buddha; and Lord Kalki, who destroys all demons at the end of the millennium.
In order to deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I advent Myself millennium after millennium. (Bhagavada-gita As It Is 4.8)
full text and purport More
The Hare Krishna Movement 