The active voice indicates that the subject is acting (e.g., "I ate the sandwich").
The passive voice indicates that the subject is being acted on (e.g., "The sandwich was eaten"). Although it is often avoided for stylistic reasons, passive voice is not a grammatical error. In some types of writing passive voice is even preferred, as when the agent of an action is not wished to be specified (e.g. "mistakes were made"), but more often, writing is made more wordy, if not less clear, when the passive is used.
Some languages also have a middle voice, which indicates that the subject acts on himself or for his own benefit or that the plural subjects act on one another. Yet another voice in some languages is the antipassive voice, in which the direct object of an otherwise transitive verb is not expressed (e.g., "I ate," without specifying what the speaker ate). The antipassive voice is usually found in ergative languages and in accusative languages in which transitive verbs in the active voice agree with both subject and object. In accusative languages in which verbs do not agree with their objects, an antipassive-like construction can typically be formed simply by not mentioning the object.