Lord Sri Vishnu
Preserver of Dharma
Vishnu is universally know as the preserver in Hinduism. It is his job to keep the Hindu dharma going and to descend in avatar form during a time when the dharma gets too weak or broken.
Married to the Goddess of Fortune
His consort, or shakti, is Lakshmi the goddess of fortune. She is always with him in some form and is said to reside upon his chest as a special mark or even to be invisible and sitting on his chest or lap.
Many Avatars
He has ten main avatars and many other lesser known avatars.
Matsya is the fish avatar. He saved Manu (similar to Noah) from
a
great flood.
Kurma is the tortoise avatar. He helped the gods recover the Nectar of Immortality.
Varaha is the boar avatar. He saved the Earth from a demon that tried to drown the world.
Narasimha is the lion-man avatar. He saved his devotee Prahlada from his demon father.
Vamana is the dwarf avatar. He tricked a demon who had stolen all the universe through a boon, into returning it.
Parashurama is the axe wielding avatar. He killed off the over abundance of warriors.
Rama is an avatar that came to show an example of perfect conduct. He is greatly worshiped today.
Krishna is one of the most important avatars. He came to bring the teachings of the Bhagavad-Gita, the most important holy text of Hinduism, and also to relieve the over burden of the Earth of too many warriors by helping to push along events that lead to the Mahabharata War. He also killed many demons and cleared the way for the start of the Kaliyuga (cycle of lowest dharma). He has a huge following.
Buddha is an avatar that is familiar to most. He came to end the degraded ritualistic practices of the Brahmins, who at that point were killing animals in the name of religion due to the fallen influences of the Kaliyuga.
Kalki is an avatar yet to come. He shall come to kill off the the evil people at
the end of the Kaliyuga. This won't be for a very long time yet.