Are Lord Vishnu and Lord Krishna same?

Vishnu is one of the principal Hindu deities and the meaning of the name is the Prevader. Vishnuji has had nine avatars till now and also combines numerous lesser divine figures and local heroes chiefly centred around two major ones - Rama and Krishna.
His appearance is immeasurable and has 10 Avatar but all are not the same. Among the other names of Vishnu, the prominent are Vasudeva Shri Krishna, Narayana and Hari.