PBX stands for private branch exchange. It is essentially a private phone network for the office, a type of VoIP system with multiple phone lines that allows you to take many different calls at the same time without hosting any infrastructure onsite. It routes incoming and outgoing voice calls through the business's phone lines and offers a variety of convenient features like call holding, routing to extensions, and conference calling, which are essential for doing business.
A PBX phone system is different from a standard multi-line phone system in that the call switching and routing does not occur directly through the phone unit, but rather is centralized. This makes a PBX more easily scalable and cost-effective. A PBX system can also be hosted in the cloud (called a host PBX or virtual PBX) and handle VoIP calls (called IP-PBX). These systems are very flexible and scalable.