A composite key is one that consists of more than one attribute.
If the ER diagram contains the attribute names for each of its entities, a composite key is indicated in the ER diagram by the fact that more than one attribute name is underlined to indicate its participation in the primary key.
A composite attribute is one that can be subdivided to yield meaningful attributes for each of its components.
For example, the composite attribute CUS_NAME can be subdivided to yield the
CUS_FNAME, CUS_INITIAL, and CUS_LNAME attributes. There is no ER convention that enables us to indicate that an attribute is a composite attribute.