Simply put, a comic book inker renders another artist's pencil drawings in India ink.

Typically, in comics, there are two artists who work on any book. A penciller, who renders the original drawing in pencil; and the inker who transforms/interprets this drawing into a camera-ready piece of black-and-white art-work.

Here are a few reasons.

1.) Different kinds of skills are needed for each aspect of the job. The penciller CAN render drawings tonally (in other words: using shades of grey). In fact, it's very difficult for a penciller NOT to work tonally. The inker, however, can only use black or white. Therefore, a large body of techniques used to represent different shades of grey [and textures, moods, distance, weight, depth, et cetera] is very beneficial. In other words: while the penciller must use his (or her) creativity to imagine and realize the situations put forth by the writer in the script; the inker must use his (or her) creativity to dream up a way to represent the subjects drawn in pencil and complete the artwork in ink.

And, yes, there ARE some artists who can do both; but then one has to consider time.

2.) Limited time. A majority of the comic books on the market are published monthly. Some pencillers can pencil more than one issue of a comic book each month; but some can't. Some inkers can ink more than one full issue of a monthly book each month; but some can't. A team is more likely to turn-in work on time. (But, even then, it's no guarantee.)

3.) A second set of eyes. Sometimes a penciller has worked on a page so long that he may not perceive a drawing error he would want to change. Or she runs out of time to really finish drawing a figure. Ideally, the inker will scrutinize the page as he works and make sure everything looks right.

My advice to interns (and anyone who asks) is that an inker should never trace exactly what's already drawn on the page in pencil. An inker has to consider texture, light source, the underlying form of an object, distance from the observer, and a few other things before putting brush (or pen) to paper.

A big part of the job is "educated mind reading". Asking yourself: "what does the penciller really want when he makes this mark on the page?" Sometimes she wants a very specific final effect. Sometimes it's obvious that he doesn't know what he wants and is -in effect- saying: "go ahead! Do whatever you like." Communication with the penciller is important. When in doubt, I ask. Although I need to ask less and less as I work with a penciller more.

In general, An inker should enhance the quality (as it relates to publishing/printing, and -if possible- artistic merit) of the pencilled art-work without changing the feel or style of the work.