Q. What is the difference between Muni, Rishi and Deva Rishi? What is the difference between Naga and Sarpa?

A. Muni means a thinker. It is said that one can not be a Muni unless his opinion is different than others.
Rishi means a sage, one who is practicing the life of simplicity and renunciation.
Deva Rishi means sage amongst demigods. Sri Narada Muni is one of them.

Naga refers to Cobra, and Sarpa is a general term for Snake.