Difference Between Object & Complement

In English grammar, an object is a noun, noun phrase, or pronoun that is affected by the action of a verb (a direct object or an indirect object) or that completes the meaning of a preposition (the object of a preposition).

Here are examples of these three types of objects.

direct object
Marie wrote a poem.
(The noun poem follows the transitive verb wrote and completes the meaning of the sentence.)
indirect object
Marie sent me an email.
(The pronoun me comes after the verb sent and before the noun email, which is the direct object in this sentence. The indirect object always goes before the direct object.)

object of a preposition
Marie lives in a dorm.
(The noun dorm follows the preposition in. Together, in + a dorm form a prepositional phrase.)

The Complement can often be confused with the Object. While the Subject and Object of a clause, in the vast majority of cases, refer to different entities, the Complement gives more information about either the Subject or the Object. As with the Subject and Object elements, there is only one grouping or phrase which is considered to be the Complement of a clause.