Proprietary software is software that you are not free to use, copy, modify, and study as you like. It's a form of abuse of users, which sucks and makes people depressed; it gives suits power over creatives.
The seemingly pointless use of proprietaryhardware to achieve the same purpose as other more common or open-standard solutions.
The Apple Display Connector was a proprietarded way for Apple to make the G4 Cube not have so many cables going from the computer to the display and keyboard. It combined USB, power and video into one cable.