Galaxies are vast systems that contain stars, planets, gas, dust, and dark matter, all bound together by gravity. They come in various shapes and sizes, with the most common types being spiral, elliptical, and irregular. Our own galaxy, the Milky Way, is a spiral galaxy that houses billions of stars. The study of galaxies helps astronomers understand the universe's structure and evolution. Each galaxy can contain millions to trillions of stars, and they often form clusters. Observing distant galaxies allows scientists to learn about the universe's history and the formation of celestial bodies.