Property management systems may be used in real estate, manufacturing, logistics, intellectual property, government or hospitality accommodation management. They are computerized systems that facilitate the management of properties, personal property, equipment, including maintenance, legalities and personnel all through a single piece of software. They replaced old-fashioned, paper-based methods that tended to be both cumbersome and inefficient.

In the hospitality industry a property management system also known as a PMS is a computerized system used to manage guest bookings, online reservations, point of sale, telephone and other amenities. Hotel property management systems may interface with central reservation systems and revenue or yield management systems, front office, back office and point of sale systems.
There are many different types of property management systems available like OPERA and GUEST360, but a user/business will want to work with the different companies to determine which property management system best fits their needs( From Wikipedia )

The PBX provides a data link interface to the Property Management System (PMS) accommodating hotel management features. The PMS can be any computer connected to the PBX via RS-232c or TCP/IP interface. It communicates with the PBX using the specified protocols.

The data link interface allows the PMS to accommodate both front and back office Hotel management features by providing a means of communication between the PMS and the PBX for features such as
Maid Status - Room status information (Ex. Cleaning necessary, Cleaned, etc)

Message Waiting Lamp Status Change - Turns Message Waiting Lamp ON and OFF at the guest and administration telephone.

Station Message Detail Records (SMDR) - PBX send all Local, National & IDD call details to the PMS with Date/time, number dialed, duration and etc