What is a hormone?

A hormone is a chemical messenger. Just remembering these two words gives you a starting point to explain the properties of a hormone in as much detail as you need. We'll break it down: 

A) Chemical: a hormone is a chemical compound, normally either a protein or a steriod (lipid-based) hormone.

B) Messenger: a hormone signals from one tissue to one or more target tissues. In order to do this, it is secreted into the blood from an endocrine gland (a ductless gland). It then travels in the blood until it reaches the target tissue with receptors complimentary in shape to the hormone molecule. The act of the hormone binding to the receptor initiates a cascade of reactions in the cell that alters the way in which the cell (and therefore the tissue in which it is found) functions.