A classical line printer is a printer which prints one line (of usually 132 characters) at a time.
Usually fed with tractor feed printer paper, line printers were the first high-speed printers used in data processing. Nowadays, the classical, mechanical line printer has mostly been replaced by laser printers or line matrix/shuttle matrix printers.
Principle of operation:
Data is sent from the host to the device's buffer a line at a time. A print chain (imagine a cross between a bicycle chain and the types of a typewriter) moves in front of a hammer bank with one hammer per print position. Whenever the correct letter on the chain is in front of one of the hammers, the hammer hits the character on the chain, which in turn will hit the ribbon, creating an imprint on the paper. As soon as the line is completed, paper is advanced by one line.
The whole process is rather noisy, print quality is limited and except for bolding, and underscoring there is no real variation in the (fixed-width) fonts.
Trivia:
Speed is measured in LPM (lines per minute)
lpt, lpd, lpr and similar terms all have their origin in line printer.
Form control and control characters are not incuded in this wu - yet...