Once every four weeks or so a single egg cell is released by an ovary. The tiny egg cell moves from the ovary along a tube towards a muscular organ called the womb (uterus).
Each month the lining of the womb thickens. If the egg cell is not fertilised it leaves your body. The new lining of the womb then changes shape and becomes thinner. A small amount of blood seeps very slowly out of your body through your vagina. This is called a period. On average you loose about two tablespoons of blood each period.
The length of a period is different for everyone. Don't worry if your friend's lasts for a week while your only last for a couple of days or the other way around. The age at which you first start your periods also varies. The most usual time is about a year after your breasts begin to grow.
For the first year or so many of you may find that your period doesn't happen exactly every four weeks. This usually settles into a more regular pattern.