What is active voice and passive voice? What is the need of passive voice?

Thanks for the invite, I apologize for the answer, I leave the answer for the professionals

A sentence with active subject is active voice.

Example: Forces killed the notorious killer in a raid. 

A sentence becomes passive voice when its subject becomes passive.

Example: The notorious killer has been killed by forces in a raid.

When object needs to be highlighted more than subject we use passive tense.

these are type of grammar we use constructing a sentence. In active voice subject is something or someone that does an action. whereas in passive voice it is object that does an action. 

thanks for the invitation

 I agree with specialties answers

Active Voice is the usual structure for statements: SVO = subject    verb    object

Passive is restructuring this statement to be : OVS

We use it when the doer of the action is : unknown, logical, less important than the receiver of the action, or when we want to conceal it

The passive verb =  Be (in the tense of the sentence) + p.p.

The agent (doer) is preceded by (by)

He called her >>> She was called by him

If I remember correctly, this question has been raised in this forum previously, but nevertheless, here is my take. Voice represents the state of the subject in the sentence: if the subject is actively performing the action, the sentence is called in active voice, whereas, if the subject is not performing the action, or rather is the passive recipient of the action, the sentence is called to be in passive voice.

1. I ate lunch. (Active voice)

I = subject

ate = verb

lunch = object 

2. Lunch was eaten by me. (Passive voice)

You see, lunch, which is the object in active voice, takes the place of subject in passive voice.

Why? What is the need of passive voice sentences? The main reason is also the same, that is, to make the doer of the action passive. When one thinks that the doer of the action is not as important as the object upon which the action is done, one uses passive voice.

3. Smoking is prohibited here.

4. Plastics are banned in my state.

5. The plane was hijacked.

It is not more important who prohibits smoking than the prohibition of smoking. Similarly, ban on plastics and hijacking of the plane are more important to the speaker (mind you, to the speaker) than who banned, or who hijacked the plane. If, to someone else the doers are more important, s/he may say:

6. The government in my state ban plastics.

7. Some terrorists hijacked the plane. 

Newspapers are notorious for using passive voice, especially for the headlines.

I agree with specialties answers, thanks for the invite. 

Active voice describes a sentence where the subject performs the action stated by the verb. In passive voice sentences, the subject is acted upon by the verb.

Sentences written in the active voice are easier to understand than sentences written in the passive voice. Switching the passive voice into the active voice is straightforward, but it requires a bit of practice. In the equivalency table below, notice that the tense of the verb to be in the passive voice is always the same as the tense of the main verb in the active voice. In order to use the active voice, you will have to make the subject of the action explicit.