Asterisk is an open source Voice Over Internet Protocol (VOIP) software. Which turns a computer into a complete VOIP Phone PBX system.
App_rpt is an open source modified version of the Asterisk software. It uses Radio Over Internet Protocol (R.O.I.P) to inter-connect radios, repeaters, and amateur radio operators globally. When amateur radio operators refer to Asterisk, they are referring to the app_rpt version of Asterisk.
The AllStar Link Network Provides Amateur Radio Operators a way to access a repeater or simplex radio known as Nodes via a computer with a speaker and mic or through a phone. Nodes are on an app_rpt modified Asterisk server.
In simple terms, a repeater or radio can be linked to one or more repeaters and radios via the internet. These internet linked repeaters can then be accessed using standard FM radios if in range of a local node or using software on a PC, smartphone etc.