What Character's Own
This category and the next are the two that present the most inconsistencies when writing. If your character loses his dagger in scene five, yet slices his enemy with it in scene eight, that break in consistency causes the reader to falter wondering if they missed something.
Tracking possessions prevents consistency glitches. You don't want to lose the reader as they flip back searching for what they missed. Track what your character owns and wears. If they have on boots that clack on the hard tile floor in chapter 1, unless they've changed they shouldn't be sneaking up wearing moccasins in chapter 4. Or if they lose their jacket, don't make it available when the temperatures drop. What characters own and use impacts logic.
Donna Sundblad's writing book, Pumping Your Muse recommends keeping a running list to track pertinent items owned or used by each character. When a chapter is written, read through it and update your tracking lists. This allows you to catch inconsistencies when the plot heads in a new or unexpected direction. Tracking allows wardrobe and possessions to keep up with each change.
Character Physical Traits
Characters evolve. Tracking their physical attributes provides a quick reference for novelists in the throes of emerging plots threads. Don't stop to wonder what color eyes-tracking allows the author to glance at the list, add the detail and keep writing.
Tracking physical characteristics also helps to avoid a blue-eyed character from becoming a green-eyed vixen later in the story just because green eyes suit her personality at that point. It's fine to make the change, but tracking not only follows the changes, but forces the writer to go back and search for any mention of the physical attribute in earlier chapters. It's necessary to hunt down previous references making them constant throughout. The goal is consistency.
Traits and Quirks
Real people have quirks. Adding small habits and quirky mannerisms brings realism to your characters. Do they bite their nails? Twirl the hair at the back of their neck? Stand with slumped shoulders?
Such traits help to paint a personality. Personalities fluctuate but habits die hard. Track habits and quirks and keep them consistent.
Guest Author Bio: Bryn Colvin is an author of fantasy and erotica books. She is largely published electronically. When not writing, Bryn is an avid reader, an active pagan and a folk enthusiast.