When you “inform someone of” something what you are doing is telling him something. You are providing the individual certain facts about something.
Here are a few examples. *Brijesh was informed of the Board’s decision. *The students were informed of the tragedy.
When you “inform on” someone what you are doing is providing information about the individual to the police or people in authority. This often causes problems for the person who has been informed on because people suspect that he has done something bad. The phrase sometimes carries a negative connotation with it.
*Laxman informed the police on Harish. *I have a sneaky feeling that someone has informed on us.