Basically there's 3 phases where a hair strand grows, stops growing/ rests, and then sheds. The length of time that it takes for a hair strand to grow to the time that it sheds is the growth cycle, and also determines how long hair can possibly be. For example, hair on your legs and arms have a much shorter life cycle than the hair on your head. So even if you don't shave the hair on your arms, it's never gonna be as long as the hair on your head.
It's also the reason why some people have longer hair than others. Some people have longer hair life cycles of, say 7 years, so their hair can grow down to their waist, while others with a life cycle of 5 years can grow it to their mid' back. Men have shorter hair life spans than women, so their hair is shorter. I
think caucasians have longer cycles, as well as faster growth rates.
Cathy Howse does a good job explaining this, I think, on her site:
www.ubhpublications.com