PCL, short for printer command language, consists of a large set of commands for controlling the Hewlett-Packard LaserJet and DeskJet families of printers, and compatible printers from other manufacturers. PCL commands are used to tell the printer where to place text or graphics on the page, which font to print, whether to print bold, italic or underlined text, and so on.
As HP has released new printers, PCL has steadily evolved, gaining new, increasingly sophisticated commands. Hp recognizes several versions, or levels, of PCL. PCL Level 5, which first appeared in the LaserJet III, represented a major jump in capabilities, with the ability to print scalable fonts (not just bitmapped fonts as in earlier levels) and fancy, PostScript-like graphics. All PCL printers respond identically to a common core set of PCL commands, but older printers equipped with an earlier level of PCL can't understand the commands from higher levels.
You might see the term PCL or printer command language used more generically to describe the set of commands for controlling other types of printers. In general, each major make of printer (for example, Epson, Toshiba, and Oki data) has its own command set, although many printers recognize the Epson commands.