When you place a caller on Hold, the only person that can retrieve this call is you. That call is exclusive to the person that places the call on hold.
Parking a call allows any user on the system to pick up the call from any phone on the system, either by pressing the lit Parking Lot Button or by dialing "*89", also every user will be aware that there is a call parked
Please note that a call put on hold will stay on hold until the caller hangs up. A Parked call will time out after a pre-set interval and ring to all phones (except Mobile) so that the caller will not get frustrated.
When a Parked call times out and rings all phones you will see a prefix "HLD:" on the Caller ID so that you know this caller has been holding for some time, allowing you to respond appropriately.
For example "HLD:Michael Jordan"
Below are two scenarios to help to illustrate the difference between the two
Hold: You are speaking with a caller and need to locate their file in a drawer in your desk. You place the call on Hold so that you are the only person that can retrieve that call.
Park: Your caller needs information from your coworker who is not at their desk, you place the call in Park allowing you time to locate that coworker; once you locate them they can retrieve the call themselves. If, instead of Parking the call you used Hold you would have to return to your desk to retrieve the call to transfer it to your colleague.