The Adrenergic Receptors Explained

Adrenergic receptors are receptors on the surface of cells that get activated when they bind a type of neurotransmitter called a catecholamine. Catecholamines are involved in the stimulation of our organs by the sympathetic nervous system; they help to trigger the fight or flight response. Many compounds, such as Epherdrine, Caffine (indirectly), Clenbuterol, Albuterol, Yohimbine, etc, work by interacting with the adrenergic receptors. I found this video very helpful in understanding these receptors and how various compound exert their effects

Further reading for each specific receptor (wikipedia)
Alpha-1: https://en.wikipedia.org/wiki/Alpha-1_adrenergic_receptor
Alpha-2: https://en.wikipedia.org/wiki/Alpha-2_adrenergic_receptor
Beta-1: https://en.wikipedia.org/wiki/Beta-1_adrenergic_receptor
Beta-2: https://en.wikipedia.org/wiki/Beta-2_adrenergic_receptor
Beta-3: https://en.wikipedia.org/wiki/Beta-3_adrenergic_receptor