Active and passive voice sounds like something complex and complicated, but really, it's not. It's about the relation existing between the subject and the action of a sentence, more exactly about who does what. But the best way to understand these is through relevant examples so let's go for those!
Active Voice vs. Passive Voice
In active voice, the subject acts upon the verb. Active voice presents how a subject does the action that a third supports, the object of the message. Passive voice, on the other hand, presents how a subject is the recipient of the action of the verb. Or more exactly, the subject is also the one that supports the action of the message.
If we were to explain this another way, active voice is that where the grammatical subject from the sentence is also the logical subject, the one that does the action, whereas in passive voice, the grammatical subject of the sentence is different than the logical subject. In passive voice, the grammatical subject is actually the one that supports the action done by the logical subject. Still foggy and unsure? The examples below will clear these confusions away for you!
When do we use active voice?
Active voice is the one more preferred in daily conversations, in letters and actually in any kind of writing. It is considered to be more logical, fluent, more readable and more elegant. Active voice creates a direct, strong message, easily comprehensible by the audience because it's presented straightforward.
Example: The girl washed her dog. - in active voice, the logical subject is the same with the grammatical subject of the sentence - the girl, and it is showed how she does the action of washing, which is supported by a third party, by the dog.
When do we use passive voice?
Now passive voice is often considered wrong or excessive. Though this is not actually a matter of correctitude, but a matter of readability. More subtle, weaker, less direct and fluent, passive voice is considered to be more difficult to understand and read. So it's recommended to use passive voice in moderation. There are, anyway, a few words that you'll most often find in passive voice, rather than active - we'll exemplify them right away.
But before that, let's take the same example as above and to take it from active to passive voice so you clearly understand the difference.
Example 1: The dog was washed by the girl. - in passive voice, the logical subject of the sentence is not the same with the grammatical one. Here, the grammatical subject is the dog, but it's also the one that supports the action made by a third. Who does the action here? The girl is actually the one that actively washes the dog, which makes "the girl" the logical subject of the sentence.
Example 2: You are supposed to listen to your mother. - as already mentioned, some verbs are more likely to be used in passive voice than active. Just as in this example. The verb "to suppose" will most frequently be used in passive voice, in the form "to be supposed to". The same applies for "expected to", "scheduled to", etc.
After you get the difference between active and passive voice, there is nothing difficult about them anymore. It's simply about the way you choose to express your message: more directly or subtler. If we were to resume everything in the shortest possible explanation, passive voice has a passive grammatical subject, that supports the action expressed by the verb and pursued by a third person (the logical subject), whereas active voice is directly expressing the way the grammatical subject (the same with the logical subject) of a sentence pursues an action supported by a third party (the object of the sentence).