A copula, (also called a linking verb), is a word that is used to link the subject of a sentence with a predicate. It serves to associate the subject with a predicate that could not stand by itself. For example, in the sentence, "Bob runs"( 's' is personal suffix of the verb 'run' that agrees with a subject), the predicate 'run' is already a verb, so it can be used by itself. In the sentences "Bob is old", however, the predicate is an adjective, so the linking verb 'is' (a form of 'be') is necessary and serves to associate the adjective 'old' with Bob.
I hope it helps,
Originally Posted by fredo733