When you "say" something, the words come out of your mouth. It's talking.it can be anything. When you "tell", you are doing something more specific. You are directing your words at specific people for a purpose, like "telling a story" or "telling some to do something." If we consider only oral communication, you can say things without telling, but you can't tell without saying.

Two main differences: A. meaning - "to say" is just to utter some words - "to tell" is to communicate information to someone B. usage - "say" can take a clause without going through an object. Example: He said that John was a liar. - "tell" cannot take a clause without going through an object. Example: He told ME that John was a liar. It is not correct to say, He told that John was a liar.