VoIP – Voice over Internet Protocol. This is an umbrella term for any form of voice communications where voice is carried over any IP network. This could, for example, be over your internal office data network, or over the Internet.
IP PBX – Remarkably similar to a conventional PBX, an IP PBX sends voice over an IP network (ie. using VoIP) rather than the PSTN.
Hosted VoIP/IP PBX – Your broadband connection carries voice traffic alongside data traffic, and extra PBX functionality (for example, voicemail) is held externally by a third party.
SIP/IAX/VoIP trunks – Use your broadband connection to make calls, often reducing the cost. Can be used in conjunction with an IP PBX for the “best of both worlds” – high quality calls for every day use while using trunks to save on international call charges.