TRUTH ABOUT AVATARS

"Whenever unrighteousness increased on earth, for the protection of the good and destruction of the evil, and for the establishment of righteousness there came Avatars." But who actually were these Avatars? God in human form or humans chosen by God? While the major part of the world has the concept of 'prophets' as the 'holy teachers'/'saviours', whereas the sub-continent has the concept of 'rishis' and 'avatars' as the 'holy teachers' and 'saviours'. So, can the holy prophets be the same as rishis and avatars? It would be interesting to know the connection between them as that would help us to get the true picture regarding the concept of avatars.