Active voice: the actor (whomever or whatever is performing an action in the sentence) is the subject of the sentence.
"I threw the ball."
"My dad paid the bills."
"My brother wrecked the car."
Passive voice: the object of the action in the sentence is the subject of the sentence, and the actor is not.
"The ball was thrown by me."
"The bills were paid by my dad."
"The car was wrecked by my brother."
There may be other small differences in those sentences, which would be chosen by the writer, but would have no impact on voice.
My opinion: because passive voice is nearly always more wordy than active, yet gains you little in most cases (of course there are exceptions) in terms of reader understanding, I personally seldom use it at all. But your mileage may vary, and when it does it'll be based on what you want the focus of your sentence to be: if it's the actor, use active; if it's the object acted upon, use passive. Make your decisions based on the relative importance of the actor and the object acted upon.