Puberty is the name for the time when your body begins to develop and change as you move from kid to adult. We're talking about stuff like girls developing breasts and boys starting to look more like men. During puberty, your body will grow faster than at any other time in your life, except for when you were a baby.

Usually, puberty starts between ages 8 and 13 in girls and ages 9 and 15 in boys. This wide range in ages may help explain why some of your friends still look like young kids whereas others look more like adults.

When your body is ready to begin puberty, your pituitary  releases special hormones. Depending on whether you're a boy or a girl, these hormones go to work on different parts of the body.

Source: http://kidshealth.org/