Custom is often referred to as a folkway. But there is a difference between the two that the folkways are of more general and wider character than the customs and cover all those modes of behavior or spontaneous usages which are not included in the term customs. Thus for example-shaking hands, eating four meals etc are folkways rather than of customs.
Customs are related to the survival and growth of the group but folkways are not necessarily so related. They are not made obligatory by the group. They are sanctioned informally.