When a woman start puberty her ovaries produce a egg cell every 28 days.
Day 1-5: the first 5 days of the menstrual cycle blood and tissue pass out of the vagina, it's called menstruation.
Day 6-14: the lining of the uterus thickens to receive an egg.
Day 14-28: an egg travel down from a fallopian tube to the uterus, if it isn't fertilised it leaves the body with blood and tissue. Next the cycle starts again