The front two arms signify the lord’s activity in the physical world and the two back arms signify his activity in the spiritual world.
A conch (upper left hand) indicates his communication with his devotees – reminds them to live in this world with kindness and compassion towards all living beings.
A chakra (upper right hand) is a weapon that is used against evil.
A mace (lower left hand) denotes energy and that he sustains the world by the energy he holds.
Front right hand usually is depicted as showing grace to his devotees.
He is usually pictured standing on a thousand-headed snake with its hoods open over his head. This signifies the innumerable desires and passions of the individual, which would ultimately destroy the world by possessiveness. This shows that Vishnu has controlled all desires.
Blue sky – infinite universe.
It is said that Vishnu comes down to the earth in times of distress to maintain order – these incarnations of the god are known as avatars.