When someone is `informed of’ something, he is formally told about the facts; the information is passed on to him.
Examples: 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 give information about the individual to the authorities. In other words, you tattle on the person. The people in power may use the information you provide against the person.
Here are a few examples. *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.