Take a look: http://www.differencebetween.com/difference-between-wish-and-vs-want/

"Mănâncă" is the 2nd person singular imperative mood of the verb "a mânca" = "to eat." However, I believe that you can also say "Mănânci ce vrei."

I'm not too sure about the difference in nuance. The first one sounds more like a command or invitation, the second one more like "Eat whatever you want."