The term puberty is used for all the physical changes that take place during your teens,leading to sexual maturity.
The first sign is normally the growth spurt: becoming much taller in a short time.
First your hands and feet start to grow, making them seem disproportional. Your arms and legs start to grow before your trunk, which may make you feel unattractive and clumsy for a while.
Girls start puberty about two years earlier than guys, making them look more mature. This difference is normally cancelled out by age 15 or 16.
Many guys and girls get pimples during their teens. Guys may also develop breast tissue (gynaecomastia), which is normal but may be embarrassing.

Female puberty
Male puberty

See also: voice breaking, body hair of guys and girls, wet dreams, pubic hair, striae, sebaceous glands, sweating and spontaneous erections