The sequence of PCL commands needed to produce a certain printed page is typically produced by the printer driver, a piece of software belonging to the operating system or application program that does the printing. This command sequence is then sent to the printer which interprets it and produces the page.
In PCL, commands exist to choose a certain font (which is stored inside the printer), to position the cursor on the page, to transmit a raster image to the printer, etc. These commands take the form of escape sequences: strings of characters that start with the escape character. Newer versions of PCL have an escape sequence to switch on HP-GL mode, which allows for the transmission of vector graphics.
The complexity of PCL is somewhere between ASCII (which only allows the simplest printer control, such as carriage return and line feed) and PostScript (which is a fully featured programming language and requires a sophisticated interpreter inside the printer).