A bronchus is an airway within the lungs, and its function is to carry air through the lung tissue. There are two primary bronchi, which split into the right and left lungs at the base of the trachea.
These airways then branch into smaller passages as they get deeper into the tissue of the lungs. These smaller passages are called secondary and tertiary bronchi, and they eventually branch into the much smaller passages of the bronchioles. The bronchioles terminate in little bunches of alveoli, or air sacs, where the main function of the lungs -- the exchange of carbon dioxide (CO2) for oxygen (O2) -- occurs.
Each system of bronchi can be visualized as a tree, with the primary bronchus as the trunk, the secondary and tertiary bronchi as branches and the bronchioles and alveoli as the leaves.