The Printer is a device added by Computercraft. It prints any text file from a Computer or Advanced Computer by using menu (ctrl) -> print. It must be directly adjacent to a computer or advanced computer in order to be used.
A printer can be used to print a document onto paper. In order to do this, the player must place paper (printed or not) into the top slots and any color dye (such as lapis lazuli, ink sac) into the left slot. The computer will print in whichever color that the dye is (blue from lapis, black from ink sac).
The printer will re-print on any already printed paper, as well as printing on blank paper. Printed paper's name will change from Paper to whatever the file's name that was printed is. A very interesting document could be created using different color dyes and reprinting.