Lord Vishnu took the form of Matsya and saved Manu from the Great flood at the beginning of Satya Yuga.
Kurma
God Vishnu appeared in the form of a giant tortoise when Mandrachala, the huge mountain was about to drown in the ocean.
Varaha
In Varaha Avatar Vishnu came in the form of a wild boar put the Earth to its original location.
Narasimha
Lord Vishnu took Narasimha Avatar to kill demon Hiranyakashyap.
Vamana
When demon-king Bali captured three different worlds Lord Vishnu incarnated as Brahmin known as Vamana.
Parashurama
Parashurama was the sixth incarnation of Lord Vishnu to end the autocratic rule of unrighteous kings.
Lord Rama
Ram is another avatar of Lord Vishnu who appeared to kill the demon-king Ravana.
Balarama
Balaram was the protector of Lord Krishna and helped in finding peace in their lives.
Krishna
In the Dwapara yuga, Lord Vishnu incarnated as Lord Krishna to kill his maternal uncle Kansa.
Kalki
The last incarnation of Lord Vishnu is yet to appear. There is a belief that the lord Kalki will appear on a white horse with a mighty sword to destroy theevil in the Kali Yuga.