The ferrule ("ferr-rule") is the white section of the stick supporting the tip that cushions the force of impact, protecting the shaft from splintering, warping or breaking. The best ferrules are solid plastic (or ivory on antique cues) affixed to the top of the shaft.
Ferrules are smooth and slide through the fingers easily. This helps the billiards stroke a surprising amount, for many players pull the ferrule as far back as their bridge hand on their backswing.
Their bright color assists the player as a visual aiming aid. The small, white ferrule moving back and forth is one of the most common sights in pool, so a few pros and hustlers paint their ferrule bright red or some other color, for amusement or to divert their competition. The owner adjusts to the change quickly, but opponents lose their nerve at the peculiar sight.
Inferior cues are made with ferrules that are hollow inside and slightly wider than the shaft. They slide over the shaft for play, making them simple to replace. Avoid such cues like the plague! They absorb poorly the stresses they ought to, and the stick vibrates in the player's hands during the stroke.
Ferrules can be scraped by a cube of cue chalk, which will mar and discolor the surface. Some non-abrasive toothpaste, rubbed on the ferrule with the fingers, removes most chalk stains and dirt. Wipe the surface clean gently with a cloth rag or a towel, and do not use any water on the ferrule.