The Public Switched Telephone Network (PSTN) is the concatenation of the world's public circuit-switched telephone networks, in much the same way that the Internet is the concatenation of the world's public IP-based packet-switched networks. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital, and now includes mobile as well as fixed telephones.
The basic digital circuit in the PSTN is a 64-kilobit-per-second channel, originally designed by Bell Labs, called a "DS0" or Digital Signal 0. To carry a typical phone call, the audio sound is digitized at an 8 kHz sample rate using 8-bit pulse code modulation. Multiple DS0's are multiplexed together on higher capacity circuits. 24 DS0's make a DS1 signal, which when carried on copper is the well-known T1 (the European equivalent is an E1, containing 32 64 kbit/s channels).
Many observers believe that the long term future of the PSTN is to be just one application of the Internet - however, the Internet has some way to go before this transition can be made.
Sometimes the PSTN is referred to as POTS, Plain Old Telephony System.
Note: there are also a number of large private telephone networks which are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies which are linked to the PSTN only through limited gateways, like a large PABX system.