I'd like to take WestWinds explanation a bit further. Like WW I'm from mainland Europe, but the basics should be the same everywhere else.
It doesn't really make a difference, whether the airfield is "public", "special" or else. The important factor are the hours of operation. If the AIP states the hours than that is set in stone. Outside these hours the airfield simply isn't there. In Germany landing outside operating hours is a grand felony.
No aerodrome operator can change the operating hours on his own, this has to be done via the regulating authorities and leaves a paper trail from here to Tokio.
This is where the PPR comes in. The operator now has the general permission to operate the aerodrome at times of his choosing.
Lets take the example: SAT 1000-1200, SUN 0800-1600, O/T PPR
This means the aerodrome has to be operated (read: is open) sat. and sun. at the times given. Outside these hours it can be operated, but doesn't have to. Therefore you have to check in advance.
Since most a/c are radio equipped nowadays you can use the radio to check. Let's take it you are fairly sure the abovementioned AD is open friday afternoon, you hop on your plane, drive over and call them up on the radio. If someone answers you can land, if not you don't.
Let's assume your a/c is not radio equipped (still exists) you can take the hours of our airfield for granted and land there with out any prior contact (bad style though). Outside the hours stated you have to call in advance. This time via landline, since you don't have a radio.
Let's look at those times: 0600-0800 PPR, 0800-1800, 1800-2000 PPR
The airfield has the permission to operate between 0600 and 2000, but the operator doesn't see much sense in operating in the early morning or late at night, so he restricts himself. He can operate between 0600 and 2000, however he has to operate between 0800 and 1800.
So PPR has nothing to do with radio or not, nor with details or else. Just so the operator knows you're coming, while he isn't obliged to have his airfield open.
All the above leaves emergencies, precationary landings and changes by NOTAMs aside.
Have fun,
FR