Complement

1. Definition of Complement

Complement : A word or word group that completes the predicate in a sentence.

The two kinds of complements are subject complements (which follow the verb be and other linking verbs) and object complements (which follow a direct object). If it identifies the subject, the complement is a noun or pronoun; if it describes the subject, the complement is an adjective.

2. Function of Complements

Complements are required to complete the verb, in contrast to modifiers, which are optional.

"The word 'complement' is also used in a wider sense. We often need to add something to a verb, noun, or adjective to complete its meaning. If somebody says I want, we expect to hear what he or she wants; the words the need obviously don't make sense alone; after hearing I'm interested, we may need to be told what the speaker is interested in. Words and expressions which 'complete' the meaning of a verb, noun, or adjective are also called 'complements.'

3. Example

• "Imagination is the one weapon in the war against reality."
  (Jules de Gaultier)
• "Love is an exploding cigar we willingly smoke."
  (Lynda Barry)

• I want a drink, and then I want to go home.
• Does she understand the need for secrecy?
• I'm interested in learning to fly.

Many verbs can be followed by noun complements or -ing forms with no preposition ('direct objects'). But nouns and adjectives normally need prepositions to join them to noun or -ing form complements."
(Michael Swan, Practical English Usage. Oxford Univ. Press, 1995)

from : http://grammar.about.com/od/c/g/complterm.htm