A Geiger-Müller counter is a device used to detect and measure ionizing radiation, such as alpha particles, beta particles, and gamma rays. It consists of a Geiger-Müller tube filled with gas that becomes ionized when radiation passes through it. This ionization creates a pulse of electricity, which is counted and displayed, allowing users to gauge radiation levels. These counters are commonly used in various fields, including nuclear safety, medical applications, and environmental monitoring. They are portable, easy to use, and provide immediate readings, making them essential tools for anyone working with or around radioactive materials.