Overview of LocalTalk Protocol

LocalTalk refers to the physical networking -- that means the built-in controller in many Apple computers, the cables and the expansion cards required on some systems. The "official" Apple cabling system typically uses a "bus topology" where each device in the network is directly connected to the next device in a daisy chain. The illustration on the Farallon book cover below gives an idea of how a bus looks.
Ethernet is the most-used method of Macintosh networking and all new Macs sport an Ethernet port, but the longevity of Macs mean there's still a bunch out there with serial ports (i.e., LocalTalk support) but no Ethernet. New iMacs and G4s lack serial ports, so can't network directly with older LocalTalk Macs and printers.

LocalTalk implementation utilized the Mac's RS-422 printer port with twisted-pair cabling and 3-pin DIN connectors. Systems were daisy-chained together and required adapters to work with the Mac's onboard DB-9 or 8-pin DIN connectors. LocalTalk provided a fairly speedy 230.4 kbps networking speed, very useable for file sizes and traffic levels of the day - compare this to the still used 56 kbps modem.

A variation of LocalTalk, called PhoneNet used standard unshielded twisted pair telephone wire with 6 position modular connectors (same as used in the popular RJ11 telephone connectors) connected to a PhoneNet transceiver, instead of the expensive shielded twisted-pair cable. In addition to being lower cost, PhoneNet-wired networks were more reliable due to the connections being more difficult to accidentally disconnect.
A LocalTalk-to-Ethernet Bridge is a network bridge that joins the physical layer of the AppleTalk networking used by previous generations of Apple Computer products to an Ethernet network. Some LocalTalk-to-Ethernet Bridges only performed Appletalk bridging. Others were also able to bridge other protocols. For example: TCP/IP in the form of MacIP.