Is there any difference? Any situations in which one work but not the other?
As verbs, the two can mean the same, as weevil points out: to pay specific attention to an object or action that one performs.
When that meaning is what you want to convey, the two are indeed interchangeable.
However, both verbs have other meanings as well, and they do certainly not mean the same.
I can, for instance, focus a camera (or more precisely, a lens) or I can focus a light beam (by use of a lens).
I can concentrate a solution (making it "stronger"), items can concentrate around a central point.
Focus has many more meanings, so does concentrate. But assuming you're talking about the verb to focus, and verb to concentrate. In that case, there is no difference at all, both are interchangeable as verbs, and would be used to describe the word:
Focus; verb, pay particular attention to, to concentrate on.
Concentrate; verb, focus all one's attention on a particular object or activity.