VOICE IS THAT FORM OF A VERB WHICH SHOWS WHETHER WHAT IS DENOTED BY THE SUBJECT DOES SOMETHING or HAS SOMETHING DONE TO IT.

ACTIVE AND PASSIVE VOICE :

A VERB IS IN THE ACTIVE VOICE WHEN ITS FORM SHOWS ( AS IN SENTENCE 1 BELOW) THAT THE PERSON OR THING DENOTED BY THE SUBJECT DOES SOMETHING : OR IN OTHER WORDS IS A DOER OF THE ACTION.

The Active Voice is so called because the person denoted by the Subject acts.
Compare :
1.Ram helps Hari .
2.Hari is helped by Ram.

It will be seen that these two sentences express the same meaning. But in sentence 1, form of the Verb shows that the person denoted by the subject does something.

RAMS ( THE PERSON DENOTED BY THE SUBJECT) DOES SOMETHING.
The Verb helps is said to be in the ACTIVE VOICE..

A VERB IS IN THE PASSIVE VOICE WHEN ITS FORM SHOWS ( AS IN SENTENCE 2 ABOVE) THAT SOMETHING IS DONE TO THE PERSON OR THING DENOTED BY THE SUBJECT.

The Passive voice is so called because the person or thing denoted by the Subject is not active , but passive , that is suffers or receives some action.
In sentence 2, the form of Verb shows that something is done to the person denoted by the Subject.

SOMETHING IS DONE TO HARI ( the person denoted by the Subject ).

The Verb helped is said to be in the Passive Voice .