Telling means that one person is sharing information with another person. For example, you could tell your friend a story, a global news piece, or a cooking recipe. Telling is one-way communication and requires no feedback from the other person. Teaching means that someone is instructing another person how to apply or understand knowledge or skills. Teaching generally has a more formal connotation and could involve the student practicing what he is taught. You could, for example, teach someone how to tell a story, how to analyze a news piece, or how to cook a particular recipe. Teaching often involves a demonstration by the teacher and then practice by the student. Depending on the context, "tell" or "teach" might both be acceptable. For example, I am telling you the difference between "tell" and "teach," but I am also teaching you the difference...if you learn what I am telling you!