The voice of a verb can be either active or passive. The active voice is the most common and preferred in English usage. In an active clause the subject and object of the main verb are in their usual position, i.e. SVO, 'Alex caught the thief' however, in a passive sentence, the object is transferred to the subject position, e.g. 'The thief was caught by Alex.' This can have the effect of emphasising the object or diminishing the effect of the subject. in fact, in a passive construction, the subject can be hidden completely, e.g. 'The thief was caught.'
In the active voice, 'The cat sat on the mat' becomes, in the passive voice, 'The mat was sat on by the cat'. The passive voice is used when the style demands that the object of the action is to be given emphasis over the subject by what is called fronting of the object. What makes the passive useful is that it gives the possibility of reducing or removing the reference to the subject to diminish the role of the subject in the action of the sentence. This style popular in newspaper headlines which want to sound authoritative and impressive yet not point the finger to the cause of an action, e.g. 'Woman murdered in gangland shooting'. Here the subject (called the agent, or the agency of the verb) is not even mentioned and allows a very different style of sentence to be created. The passive voice is also the style of choice for reports where the subject is not important, e.g. the person who carried out an experiment or interview is usually of far less importance than the details and results of the experiment.