At its simplest a hormone is a chemical messenger from one cell or group of cells to another.  Hormones are released (secreted) in the body and have an effect on other parts of the body.  The effect is to communicate with possibly distant parts of the body.  So for example, the adrenal glands may secrete adrenaline, which has an effect on several different organs.  An endocrine gland is a gland which secretes hormones (note that not all glands are classified as endocrine glands; for example, sweat glands or lymph glands are not endocrine glands). 

Hormones are found in all organisms with more than one cell, and so they are found in plants and animals.  They influence or control a wide range of physiological activities such as growth, development, puberty, level of alertness, sugar regulation and appetite, bone growth, etc.  You also find that problems with hormones and the way they work contribute to some of the major diseases of mankind; for example, diabetes, thyroid conditions, pituitary conditions, some sexual problems, some neurological problems, appetite and obesity, bone problems, cancer, etc.

There are whole sub-specialities devoted to specific areas where hormones work.  For example:

• Paediatric endocrinology - looking at hormones in children
   
• Thyroid endocrinology - looking at how the thyroid affects metabolism
   
• Endocrine-disrupting chemicals - where chemicals which mimic the effects of hormones are present in the environment
   
• Comparative endocrinology - which looks at the way similar hormones work in different species (eg from insects, through to fish, birds, mammals, etc)

Sometimes there are specific societies devoted to the study of these subspecialities.