@KeshavSrinivasan - found some info in blogs. -Vishnu assumes five forms:
1.The Para Form.-Paravasudeva – Para is the highest form of Vishnu found only in Sri Vaikuntha
2.The Vyuha form.Krishna,Sankarshana, Anirudda, Pradyumna: The Vyuhas are emanations from the above mentioned Para Vasudeva form, and are four in number, collectively called, Catur Vyuha Brahman
3.The Vibhava form: Apart from the Vyuhas the Lord is said to assume various manifestations, from time to time, to protect the virtuous, punish the evil-doers(wicked) and re-establish Dharma.