What are hormones?

Hormones are the chemical messengers that pass through the bloodstream from glands to cells throughout the body.

The purpose is to change the function of the destination cells/organs.

Why do glands emit hormones?

Because of some other change. For example, when the Thyroid gland (located in the base of our neck) receives iodine in food it releases two hormones thyroxine (T4) and triiodothyronine (T3). 

What do hormones do?

Hormones regulate or control many functions in our body, for example, T4 and T3 (as mentioned above) affect a lot of other cells and organs. They regulate/maintain the rate of metabolism (the rate at which calories are burned), the heartbeat, body temperature, muscle health, etc.