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What hairballs actually are
Why cats swallow hair in the first place
A cat's tongue is covered in tiny backward-slanted projections called papillae. They make the tongue an excellent comb - and a one-way street for loose hair. As Cornell's hairball guidance explains, as the cat grooms, the papillae propel shed hair down the throat and into the stomach, and the keratin that makes up hair is indigestible. Most swallowed hair travels through and leaves in the feces. Some of it stays in the stomach and mats together over time.
What a hairball looks like
Despite the name, a brought-up hairball is rarely ball-shaped. Cornell notes that hairballs passing through the narrow esophagus come out slender and cylindrical - more cigar than ball - usually around an inch long but sometimes much longer, colored by the cat's coat and darkened by food and gastric secretions. A hairball that stays in the stomach without being brought up does end up round, like a rolled-up sock. The retching that produces one is dramatic: the cat rises, retches convulsively once or twice, and deposits the clump.
Who gets them most
Long-haired breeds such as Persians and Maine Coons may be at greater risk, Cornell notes - simply because there is more hair to swallow. Cornell also notes that kittens and young cats are less apt to develop hairballs than older cats, who as experienced groomers spend a good part of their waking hours on their coats, and that hairballs develop more often in shedding seasons, when the volume of loose hair peaks. And a cat who suddenly grooms far more than usual may be telling you something else: Cornell's guidance points out that excessive self-grooming can signal a skin problem, including food-allergy-related ones, which is a conversation for your vet rather than a grooming-technique question.
When a hairball stops being ordinary
The ordinary hairball is an inconvenience. The dangerous one is an obstruction: a clump too large to pass the sphincters out of the stomach, or one that lodges in the small intestine. Cornell's guidance treats blockage as uncommon but serious - a large clump of ingested hair can block the intestinal tract and pose a deadly threat, and a detected blockage can require surgery. The signs are covered in how often is too often: repeated unproductive retching, lethargy, or a cat who stops eating or eats much less means contacting your vet promptly - Cornell advises an exam without delay, and breathing trouble is an emergency. Prevention, mostly through regular brushing, is far easier than any of that.