An agonist is a substance that activates a receptor in the body, leading to a biological response. In pharmacology, agonists can be drugs that mimic the action of naturally occurring substances, such as hormones or neurotransmitters. For example, a beta-agonist can stimulate beta-adrenergic receptors, which are involved in processes like relaxation of airway muscles. Agonists can be classified into different types, such as full agonists, which produce a maximum response, and partial agonists, which activate receptors but produce a weaker response. Understanding agonists is crucial in developing medications for various conditions, including asthma and depression.