A printer is a device used to print text on paper. Now, with the advent of laser printers, all printers can produce graphics or images, but originally, only special devices such as plotters could produce non-text output.
The very earliest printers were simply Teletypes, and other printing terminals, but there soon appeared line printers, which were specialized output-only devices which could produce high volumes of output.
There were two basic designs: 'drum printers', which had a drum which contained raised characters (similar to those in classic printing type), with a complete set arranged all the way around the drum in each printing column, and an inked ribbon run between the drum and the paper, with hammers on the other (back) side of the paper in each printing column. The drum was spun, and as the 'correct' character came opposite the hammer in each column, the hammer 'fired', and printed the character.
The more common type was the 'chain printer', where a chain with all the characters in it spun rapidly, along the line being printed, but was otherwise similar to the drum printer; as there, hammers fired to print the desired characters as they came opposite the columns in which they were wanted.