A compact disc (CD) is a flat, circular optical disc 120mm in diameter that stores digital audio data. Developed jointly by Philips and Sony, the CD was introduced commercially in 1982 and quickly revolutionized the music industry by offering a format that was smaller, more durable, and free from the surface noise inherent to vinyl records.
Audio CDs (CD-DA) store uncompressed PCM audio at 16-bit, 44.1 kHz — a specification known as Red Book audio. This provides up to 80 minutes of high-fidelity stereo sound. The data is encoded as microscopic pits on a reflective aluminum layer, read by a semiconductor laser in a CD player. Because the laser reads the disc optically without physical contact, CDs don't wear out from normal playback.
While streaming has reduced CD sales significantly, CDs remain popular with collectors and audiophiles who want to own physical media at a lower price point than vinyl. CDs also serve as an excellent source for lossless digital ripping — you can convert your CD collection into FLAC files for a personal music server, giving you the best of both worlds: physical ownership and digital convenience.
The CD's 74-minute capacity was reportedly chosen so it could hold Beethoven's Ninth Symphony in its entirety — a decision influenced by Sony VP Norio Ohga, an opera singer.