A Guide To Linux Printing - page 4
Lpd is the Linux print spooler daemon. Normally lpd starts when your machine boots and continues to run waiting to accept a printing request. Lpd can accepts jobs from either the local print client or other computers on the network. Lpd is responsible for holding print jobs until the printer is ready to accept them. Printer configuration information is stored in the file /etc/printcap. The name of the printer, the physical device that the printer is attached to, what filter to use and the settings for other more advanced options are stored in printcap. The file format of printcap is cryptic but many tools exist such as Red Hat's "printtool" that will create the file for you based on options selected from the GUI front end. Detailed information about the format of the printcap file can be found in the Linux Printing HOWTO. Once the printer becomes ready to accept a print job the spooler looks in the print queue for that printer and selects the first file waiting to be printed.