Wireframe vs. Mockup vs. Look&Feel for mobile apps?

I'm starting development of a couple of mobile apps, and I'm sometimes told to propose a wireframe or a look&feel for them, or to create a mockup. I suppose those are actually different things though I made the impression that who asks me for them is not either aware of the differences...

I'd appreciate an explanation of each of those words and their differences in a mobile development environment