Vishnu in the Cosmic Ocean
"Although Visnu is equal to Krsna, Krsna is the original source. Visnu is a part, but Krsna is the whole. This is the version given by Vedic literatures. In Brahma-samhita the example is given of an original candle which lights a second candle. Although both candles are of equal power, one is accepted as the original, and the other is said to be kindled from the original. The Visnu expansion is like the second candle. He is as powerful as Krsna, but the original Visnu is Krsna."
Teachings of Lord Caitanya, Chapter 8
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.
The expansions of Lord Krsna who come to the material creation are called avataras, or incarnations. The word avatara means "One who descends," and in this case the word specifically refers to one who descends from the spiritual sky. There are innumerable Vaikuntha planets in the spiritual sky, and from these planets the expansions of the Supreme Personality of Godhead come to this universe.
There are six types of Krsna-avataras. One is the incarnations of Visnu, called the purusa-avataras. Another is the incarnations meant for the performance of pastimes, called lila-avataras. Another is the guna-avataras, who are incarnations that control the material qualities. Another is the Manus, called manvantara-avataras, who are progenitors of the living entities. Another is the yuga-avataras, who incarnate in different millenniums. And the sixth type is the saktyavesa-avataras, who are the empowered living entities.