very simple stripped down explanation of it but it's correctNormal maps are like bump maps, except they work using more info. That's a really simplified explanation, but I think it's correct (?).
dirt map could refer to the dirtmap node in mental ray... works the same way as an occlusion, but is faster i believe. (i still dont use it, but know some people who like it.)?).
Dirt? No idea, I think dirt usually gets added just in the texture and bump maps.