A sentence is in the Active Voice when the subject of the sentence performs the action.
[Subject] + [verb] + [thing receiving action]
Salil hit the cat.
Mr. Das cleaned the terrace.
A sentence is in the Passive Voice when its form shows that something is done to the thing denoted by the subject.
[Thing receiving action] + [be] + [past participle of verb] + [by] + [thing doing action]
The cat was hit by Salil.
The terrace was cleaned by Mr. Das.