[FredL (OP)]

Fur seems to be hair that stops growing at a certain point.  How does a fur follicle know when to stop growing?  Is this the same for human eyebrows and eyelashes?  In animals, fur seems to be the rule and hair the exception.  Can hair cells become cancerous and grow out of control?

[rosy]

Apparently hair also grows to a certain length (more-or-less) and then stops growing, too - a hair follicle grows for a while, then stops for a while, then the hair falls out of it and it starts again. It's just like fur, but the length it could grow to is, for most people, longer than the length they keep their hair trimmed to (see the girls and women from that sect whose name escapes me who don't ever cut their hair and wear little triangular kerchiefs on their heads, most of them have hair that doesn't go much below the base of their spines, if so far). Different people are capable of growing different amounts of hair, some not much below shoulder level but others well down to the level of their hips.

[JnA]

Individual follicles can and do go a little rogue and produce a hair that might be two or three times the length of surrounding hair. I can't remember what causes it though..