A CD, or compact disc, is a thin, round piece of plastic about the size of your palm that stores digital information. For decades, CDs were the primary way people listened to music, played computer games, and installed software on their computers.

CDs work by encoding information as microscopic pits arranged in a spiral pattern on the disc's surface. A laser beam inside a CD player reads these tiny patterns and converts them back into sound or data. One CD can hold about 80 minutes of music or roughly 700 megabytes of computer data.

In the 1980s and 1990s, CDs revolutionized the music industry. They replaced vinyl records and cassette tapes because they produced clearer sound, didn't wear out from repeated playing, and let you skip instantly to any song. People built large CD collections, carefully organizing them in tower racks or binders.

Today, streaming services and digital downloads have largely replaced CDs, just as CDs once replaced earlier technologies. But CDs haven't disappeared completely. Some musicians still release albums on CD, and many people keep their old collections. Libraries often lend audiobooks and music on CD. The term CD has become so familiar that people still use it even as the technology itself becomes less common.