SQL & PL/SQL :: Difference Between Procedure And Function

I know difference between procedure and function.if we want a return value from procedure, we can have OUT parameter. Similarly with function, in addition to returning a value from function, it can also send an OUT parameter value as a return value. That means, in one or the other way we are able to get a return value from both program units. Normally, I would fill a OUT variable with error message when an exception occurs. I use this varaible,after procedure call, to detect if an exception occurred or not. Similar task can be performed by a function, it returns a true/false and a value thru OUT variables.

both program units return values in the form of OUT parameters. Where exactly should we use a function, where exactly should we use a procedure?