Fidonet is an inter-connecting file and message transport system used by BBSss before the explosion of the Internet. It was originally founded in 1984 by Tom Jennings as a means to network the "Fido" BBS software he developed. Later other BBS software that supported the same protocol was added to the net.
The Fidonet is (usually) a dialup based system, where the nodes connect to each other and exchange files according to a set protocol. Since the connections are using the same telephone lines that are used for the normal users of the BBS, the fidonet transfers only happen at special times of the day ("Netmail-Hour", typically 4:00 in the morning).
Today Fidonet supports its own standards for netmail (similar to Internet e-mail), echomail (Usenet-like news) and file transfers. Recent standards describe ways of transferring Fidonet messages and files by TCP/IP so it's possible to run Fidonet over Internet.
Some of Fidonet's echomail conferences are available in the Usenet news hierarchy and there are mail gates for exchanging messages between Internet and Fidonet.
Fidonet is organized in a tree structure, with fully automated software suites as branches and additional end users' software as leaves.
A typical software suite includes a mailer (a program that handles the physical transfer of files and messages between systems), a tosser (which packages messages to be transferred, performs basic routing sometimes acting as a compatibility layer for end user's software) and various other utilities (eg. file-compressor, mail-editor).
An end user's software includes a (message) reader in addition to the list above.
Fidonet usage has dropped a lot, but it is still particularly popular in Russia.