Link verbs are followed by a word that gives information about the subject itself.

https://learnenglish.britishcouncil.org/en/english-grammar/clause-phrase-and-sentence/verb-patterns/link-verbs http://dictionary.cambridge.org/grammar/british-grammar/about-verbs/verbs-types#verbs-types__2

http://www.bbc.co.uk/learningenglish/english/course/upper-intermediate/unit-22/session-4