Etymologically, each derives from words which refer to the same thing. Both the Latin word "mōs" and the Greek word "ἔθος" can be translated as "custom", and refer to that kind behavior which is customarily accepted among others. Thus, for a long time, terms derived from either were used equivalently, and often still are, since there isn't actually a definite difference between "ethics" and "morals".
Laymen consider "ethics" to refer to externally imposed guiding rules, since this word is often used in the context of professional ethics, with "morals" referring to personal guiding rules. In my perception, the philosophical usage of "ethics" has more of a theoretical tinge, whereas "morality" more often refers to implicit principles by which a society lives by. But again, there's no definite conceptual difference between these.