An ELT Glossary : Subject Complement / Object Complement

Definition 

A complement is a clause element which in some way equates with or relates to either the subject or object of the clause. Complements may be adjectives, reflexive pronouns or noun phrases.

Examples

a) Subject complements occur after copulative verbs :

Mary is a neurologist ( Mary = a neurologist).
John looked unhappy (John = unhappy)
The leaves are turning brown ( the leaves = brown)

Reflexive pronouns may also act as  subject complements, taking the position normally filled by a direct object after a transitive verb :

I've hurt myself (Subject complement : myself = I)


b) Object complements

I can prove you wrong  ( you = wrong)
She called him a fool  (him = a fool)


Further reading

Quirk, R. et al (1972) A Grammar of Contemporary English, Longman  Chapter Eight