There is a great art to both pencilling and inking. Inking is much more than just tracing lines. The inker has to correct any flaws of the pencils, and has to make decisions on how to ink every line - in what thickness, straight or jittered, "dead" or "alive", etc. The inker also has to decide on light and shadow, on which parts of the panel are to be black or colorized later, etc.
Inkers also work with other tools such as white-out or raster foil.
After the inking phase comes typesetting and colorization.
- More information on this topic is available at [ Wikipedia:Inker ]