Someone is formally informed of something when they are "informed of" it. He receives the information from the source.

Please be here at 2:00 o'clock tomorrow. I will inform you of my decision then.

We were informed of the change in plans last night.

When you "inform on" someone, you provide the authorities with details on the person. You tattle on the individual, in other terms. The information you give might be used against the person by others in positions of authority.

If you continue to come late like this, I'm afraid that I'll have to inform on you.

It was his own friends who informed the police on him.