LPR/LPD is a platform-independent printing protocol that runs over TCP/IP. While it was originally implemented for BSD unix, LPR has become almost universally accepted and supported.
LPR - Line Printer Remote. Refers to the process that sends jobs to the printer or print queue. This is analogous to the print client software and is often called the 'LPR client'.
LPD - Line Printer Daemon. Refers to the process that accepts print jobs from the LPR client. This is analogous to the printer or the print server software and is often called the 'LPD server'. The LPD waits for the client to send it jobs, and may be capable of handling multiple printers and print queues. Since it (usually) only has a single IP address, the print queue name must be used to specify the intended printer.