Asterisk is a free telephone Private Branch Exchange (PBX) system. This can most easily been seen as a telephone system used to connect callers and as a sort of answering and dispatching machine. (e.g. "press 1 to speak to ..." etc) To get yourself started have a look at The Asterisk Handbook (2003)
Connecting to existing systems
Connecting Asterisk to an existing telephone network. This is called a PSTN call (Public Switched Telephone Network) Asterisk can be connected to a SIP or IAX provider (like VoIPBuster) or by using a (voice-)modem-card of Zaptel-card in the Asterisk-server.