In Indian mythology the ten incarnations of Vishnu are highly spoken of as a theological concept in Vedic history. In every age Lord Vishnu descended on earth, in either animal or human form to preserve the world and mankind from increasing evil.
The first three avatars (Matsya, Kurma and Varaha avatar) which descended on Earth was during the Sat Yuga, next three (Narshimha, Vamana and Parasurma avatar) were in the Treta Yuga, seventh, eight and ninth incarnations ( Sri Rama, Krishna and Buddha avatar) appeared in the Dwapara Yuga. The tenth avatar – The Kalki Avatar (the slayer) is yet to be born and is said to appear in Kalyug Yuga.
In Bhagavad Gita (Chapter IV-7) Lord Krishna explained this shloka to Arjun-
Yada Yada hi dharmasya, glanir bhavati Bharata,
Abhyuthanam adharmasya tadatmaanam shryujaamyaham;
Paritranaaya Saadhunaam, Vinashaayaka cha dushkritam,
Dharma Sansthapanathaay, Sambhavaami Yuge Yuge
Hey, Bharat (Arjuna), Whenever faith declines, and it is under attack,
Whenever adharma (sin) is on ascendancy without control,
I re-incarnate myself with all my powers to restore faith,age after age!
The Ten Avatars or Dasha Avatar of Vishnu are very interesting and each one has its own story.