The Torah tells us that Aaron and his sons (The descendants of Levi) are the priests.
Then, why are the Cohains known as priests?
Only Aaron and his sons were chosen to be priests from among all of the tribe of Levi. Priest is simply a translation of the Hebrew word “Cohain.” From that point on all of the descendents of the sons of Aaron would be the priests, or “Cohanim” (plural of Cohain). In other words, all priests (Cohains) are Levis, but Levis, except for the descendents of Aaron, are not Cohains.