UUCP was a protocol commonly used on non-networked UNIX machines in the past. Similar to what Fidonet was for DOS-based BBSes, UUCP enabled UNIX machines to regularly dial up and connect to each other to form an ad-hoc network. For years this was widely used both for mail and for USENET, as well as just for transferring files from one machine to another. The Internet basically eradicated UUCP.

If you look through old code and documentation, you will see e-mail addresses given in the old bang path UUCP-style, where the names of connected machines are successively named, culminating with the name of the user on the last machine, i.e. darkstar!walrus!bayport!gertrude would send a message to gertrude on the machine bayport, first through darkstar, then to walrus, and lastly to its destination. In those times, it is easy to see how it might take days for a message to get from one end of the loose-knit network to the other depending on how often the nodes synced.

Where UUCP is used at all today, it is generally used as a gateway from a non-networked system to an Internet networked system.