Hindus believe that each human being has a soul. Therefore every person and every living thing contains God within them. If a person was perfect then he/she would become God. This is how avatars or incarnations came about. It is said that when there is a lot of evil in the world then God takes birth on Earth to banish all evil and save us all. There are reputed to be 10 avatars of Vishnu. These are :
Parasurama(Rama with the Axe)
Rama(hero of the Ramayana)
Balarama(Rama with the Plough)
Krishna(hero of the Mahabharata)
Kalki(a promised future avatara of Vishnu riding a horse).