A potassium channel is a type of protein found in cell membranes that allows the selective passage of potassium ions (K+) in and out of cells. These channels play a crucial role in maintaining the cell's electrical balance and are essential for various physiological processes, including nerve impulse transmission and muscle contraction. Potassium channels can be classified into different types based on their structure and function, such as voltage-gated potassium channels and inward-rectifier potassium channels. They open and close in response to changes in voltage or other signals, helping regulate the flow of potassium ions and contributing to the overall excitability of cells.