Simple examples:

ACTIVE:

John broke the cup.

PASSIVE:

The cup was broken.

The cup was broken by John.

Clear? Any further questions?

I bought a book yesterday. (There are 2 nouns-the first is I and the second is A book)-it's active,because there is someone who bought the book.

But when we don't say about the person who did something, it\ll be passive. for example: the book was bought yesterday. (in this example we don't know WHO  did the action but we know that there was an action with the BOOK. 

In the first example A PERSON played main role, in the first example A BOOk. 

Hope you understand everything

Thank you so much Mrs. Ramida and Mr.Berneddi M.Ed. I understand now ^_^