If you check the Merriam-Webster dictionary, it says that a teacher is "one that teaches". A guru is "a teacher and especially intellectual guide in matters of fundamental concern" or "a person with knowledge or expertise". I suppose the difference is that anybody could be a teacher. A teacher doesn't actually have to be good at what he does. A guru, on the other hand, must be an expert in his field and he doesn't necessarily even need to teach. That just depends on the definition you subscribe to.