Krishna and Vishnu
This is certainly a popular question. Here's one of the most helpful answers we've found, from Teachings of Lord Chaitanya, Chapter 8:
"Although Vishnu is equal to Krishna, Krishna is the original source. Vishnu is a part, but Krishna 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 Vishnu expansion is like the second candle. He is as powerful as Krishna, but the original Vishnu is Krishna. Brahma and Lord Shiva are obedient servants of the Supreme Lord, and the Supreme Lord as Vishnu is an expansion of Krishna."