Definition: Asterisk PBX

Open source PBX software that runs under Linux and other Unix variants on a variety of hardware, including x86, PowerPC, POWER and Xscale. Supporting traditional telephony (TDM) as well as voice over IP (VoIP), Asterisk is written in C and uses the Unix standard POSIX programming interface to communicate with the operating system.

The IAX Protocol
Asterisk uses its proprietary Inter-Asterisk Exchange (IAX) protocol to initiate calls and send the data. Supporting any type of codec, IAX can be used for video as well as VoIP. All signaling and data ride in UDP packets, and IAX can multiplex multiple data streams between servers (port aggregation).

From Digium Et Al
Developed by Mark Spencer of Digium, Inc. and maintained by the company along with the Asterisk community, a version for Windows that supported only VoIP was developed as a community project, but "AstWind" was later abandoned. Digium also offers a variety of analog and digital telephony interface cards that plug into PCI slots.