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.
Hormones are found in all organisms with more than one cell, and so they are found in plants and animals. 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). So for example, the adrenal glands may secrete adrenaline, which has an effect on several different organs. The effect is to communicate with possibly distant parts of the body. Hormones are released (secreted) in the body and have an effect on other parts of the body.
At its simplest a hormone is a chemical messenger from one cell or group of cells to another.
