Asterisk is a software that use to handle voice communications. It is created in 1999 by Mark Spencer of Digium.
Normally its installed as a server and attached telephones to make calls to one another, and to connect to other telephone services including the public switched telephone network (PSTN) and Voice over Internet Protocol (VoIP) services.
IP PBX systems, VoIP gateways and conference servers are same of application that we can develop using Asterisk. It is used by small businesses, large businesses, call centers, carriers and governments worldwide. I’m not going to describe about these application but if you are not familiar with this application, then you can simply search them on Wikipedia.
It is a free opensource software and originally design for Linux. But now Asterisk also runs on a variety of different operating systems including NetBSD, OpenBSD, FreeBSD, Mac OS X, and Solaris. And it is available for Microsoft Windows as AsteriskWin32. But still it is most stable on Linux and recommended OSs are Debian, Fedora, CentOS. And also other Linux OSs like Gentoo, SuSE, Mandrake and Slackware.