Action verbs come in two varieties: transitive and intransitive. A transitive verb has the intrinsic ability to attach directly to a noun, and that noun is called the direct object. Thus, in the sentence John hit the ball, the word ball is the direct object of the transitive verb hit.
Also, objects of verbs show up in verbal phrases as well. If the verb is transitive, it can pick up a noun and produce a phrase. Thus, in the sentence He wanted to hit the ball, the word ball serves as the object of the infinitive to hit.
If a personal pronoun takes the place of a direct object or object in a verbal phrase, it must appear in the objective case. Thus, in the sentence John hit him, the word him serves as the direct object; the word him appears in the objective case.