Performers are like...Chris Brown. He doesn't write his music, he doesn't play an instrument, and he barely sings on stage (instead just dancing and lipsynching most of the time).
Musicians write and play and do more than just entertain on stage.

Performers only perform.
Musicians also write their own music and are in my opinion much more talented.

My personal opinion is that a musician is somebody who's related to music completly , like writing it , composing it etc. etc. a performer is just somebody who performs songs or music writen by somebody else in music's case . (:

I feel that anyone who can write music and/or professionally employed musical person is a musician. Chris Brown doesn't write ALL of his music, but he can write music, even if it's just lyrics. (I don't know exactly how much or how he writes)
I don't think it has anything to do with instruments either. (Singing isn't music?)
Opera singers don't usually play instruments on stage or write the music they are performing, but I would definitely consider them musicians.
The members of the Chicago Philharmonic Orchestra don't write the music they play, but I'd still consider them musicians.

Technically, a performer can be a musician, actor, politician, someone who gives speeches. A musician is someone who writes, sings, plays, or performs music. Make of that what you will.