A penciller is a member of a comic book or comic strip's creative team. The penciller is historically considered the primary artist on a piece, although more specialised techniques in inking and colouring has levelled the playing field between these artists in the 21st century.
Nevertheless, the penciller is the one who actually draws the comic, thereby providing the template from which the colourist, inker, and letterer work.
History of the term
The term derives from the mostly historical production process in which the artwork was literally completed in pencils. The inker would then add ink to the pencils. If the piece were monochromatic, it would then be finished, save for lettering. If it were in colour, it would then move on to the colourist, prior to being lettered.
In the 21st century, comics are more rarely physically pencilled, but instead drawn with Photoshop or other graphics manipulation programmes.
Inking was not a separately-credited talent in comics for many decades. Indeed, many pencillers inked their own work. However, as inking techniques became more sophisticated, some artists specialised in inking, and the credit became more common. In the 21st century, it is comparatively rare for a penciller to be his or her own inker.
In Doctor Who comics
Prior to Ridgway's breakthrough, inkers weren't credited for two main reasons. First, Polystyle's TV Comic and Countdown comics — as well as the comics in Doctor Who annuals, were usually painted, rather than drawn. Thus, there literally were no pencillers for about the first fifteen years of Doctor Who comic history.
This tradition continued during the first year of the Sixth Doctor's era, until finally John Ridgway handed over inking chores on Profit of Doom to Perkins. While there were still occasional strips inked by the penciller after Profit, inking credits became much more common from that point forward.