Why is it that the hair on your head grows continuously but hair on your arms and legs doesn't?

Why do you think it is not? It is only worn down to an almost fixed length by our clothes and movements.
The hair on my arms grows longer summertime with lighter dress.

The hair on your head does not grow continuously, it has phases of growth and rest though it is perhaps less visable on the head and not all hairs are growing or resting at the same time. Hairs in the armpits will grow at a different rate to the legs due to the difference in bloodflow and subsequent production of dead cells.

Why is it that my hair has stopped growing on my head but the rest of it is still growing?

The effects of androgens continue long after puberty. Some areas of the skin respond to these hormones more vigorously
than others do, and at different times of life. Pubic and armpit hair begins to grow at or soon after puberty. Most men's beards do not grow strongly until the owners reach their thirties. The growth of chest hair reaches a peak even later, and hair in the nose and ears grows most in late middle age.
http://www.pg.com/science/haircare/hair_twh_24.htm;jsessionid=0MUBTOHAER0FZQFIAS... copy

My hairs starting to grow wavy.

As in waving goodbye to my head, but my beard is now growing at a faster rate. I used to get away with shaving every other day as blond hair is harder to see, but now need to shave every day.