A CD player plays Audio Compact Disks. Some support other formats as well, such as MP3 or SACD.
A CD player has two main parts: the transport, which spins the disc and reads the digital data stored on it; and the DAC, which buffers the digital data from the transport and transforms it back into an analogue waveform (music) which can be amplified. There are also systems related to error correction, control and analogue preamplification. Most CD players feature a digital output - optical and coaxial RCA are the most common - as well as analogue outputs; this allows the internal DAC to be bypassed and an external DAC to be used.