What are Hormones?
Answer:
Hormones are essentially chemical couriers which
transfer messages between cells through the bloodstream.  All multi-celled organisms including plants produce hormones.  Hormones help to regulate cell functions that have receptors for the specific hormones.  The actual behavior of the target cells depends upon several things, including the particular secretion by the hormone, and the nature of response by the recipient cells or tissue.

Some of the most popularly recognized hormones in animals are the ones that are produced by the endocrine systems of vertebrate animals.  However, hormones are produced by just about all organ systems and tissue in multi-celled organisms. 

Some variations in the methods of hormone introduction exist across the systems of multi-celled organisms; i.e., while endocrine system hormones are introduced directly into the bloodstream, the exocrine system (includes pancreas, mammary glands, liver, stomach, etc.) hormones are introduced into ducts, from which they flow into the bloodstream or directly between cells.

Some common hormones and their principal effects are:

Melatonin -- an antioxidant, induces drowsiness/sleep
Serotonin -- regulates appetite, sleep, and mood
Epinephrine (adrenalin) -- prepares fight/flight by increasing oxygen and sugar supplies to the brain and muscles, and also affects other body systems in conjunction with the fight/flight mechanism.
Growth Hormone -- stimulates cell production and overall growth
Insulin -- regulates glucose (etc.) metabolism
Oxytocin -- stimulates the release of breast milk; involved in female sexual pleasure mechanisms
Testosterone -- associated with males, stimulates growth of muscle mass; involved in puberty (i.e., voice change, etc.)
Histamine -- stimulates digestive acid secretions

There are dozens more hormones, all of which have a specific mission: to deliver chemical messages and triggers to the various cells, tissues and systems within organisms, to ensure the organisms are prepared and primed with the necessary impetus to carry out the myriad purposes of those messages. 

Without hormones, multi-celled organisms would not be able to live; it's as simple as that!
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