Biting your nails isn't as unhealthy a habit as snacking, but it's still a bad one. It's embarrassing to have ragged, chewed nails. And since it's a habit associated with anxiety, it can feel like people learn all about your mental state just by glancing at your hands.
Interestingly, the fifth edition of the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders classifies nail biting as a body-focused repetitive behavior disorder, along with hair twirling and skin picking and relates them all to behaviors characteristic of obsessive-compulsive disorder (OCD) [source: Grohol]. One big difference is that most people with OCD want to stop their practices because they don't get any pleasure from obsessively lining up their shoes or washing their hands. Nail biters, on the other hand, usually find gnawing on their tips pleasurable and stress-relieving [source: Standen].
While nail biting is not generally harmful, if you attack the cuticles you can risk bleeding and bacterial infection. One way to stop the habit is to keep your nails impeccably manicured, so you won't want to ruin them. Some people put bitter-tasting polish or even a bandage on a finger to remind themselves to stop the biting and find something else to do.