There are two kinds of subject complements.
A predicate nominative is a noun or pronoun which follows a linking verb and refers to the subject.
Example: Carla is the leader. [The noun leader follows the linking verb is and refers to the subject Carla].
A predicate adjective is an adjective which follows a linking verb and describes the subject of the verb.
Example: The buildings are very tall. [The adjective tall follows the linking verb are and describes the subject trees].