Inform of and inform on

Inform on means:

to furnish incriminating evidence about (someone) to an authority, prosecuting officer, etc.: 

He informed on his accomplices.

Inform of means:

to give knowledge of a fact.

He informed me of their arrival.

Hope it helps.

THe slight difference between these two phrases implies the direction of information..

To be informed of - means person A was told something from person B

To inform on - means person B told person A something.

I was informed of John's actions.

Dave informed on John's actions.

Neither of those sentences are the best way to put it (they sounds very odd to me), but I hope it made the meaning clearer.