Getting into PL/pgSQL…

I'm still learning the syntax. Sometimes, I'd run into this error message:

> 
> ERROR: cannot use RETURN QUERY in a non-SETOF function
> 
> 
> 

This sounds sort of cryptic and I could not find information in the Postgres documentation. Hence the question:

* What's a non-SETOF function?

And likewise, assuming there's such a thing, what's a SETOF function?