PCM (pulse-code modulation)
The plainest way to store sound digitally: measure the wave's amplitude at regular intervals and write each measurement down as an integer.
Two numbers decide everything: how often you measure (the sample rate) and how many steps each measurement gets (the bit depth). Alec Reeves patented the idea in 1938 while working for ITT in France, and its first serious use was SIGSALY, the encrypted telephone link between Churchill and Roosevelt. The phone network moved to 8 kHz and 8 bits in the 1960s, and the CD fixed what still feels like the default in 1982: 44.1 kHz, 16 bits, stereo.
Linear PCM, or LPCM, is what sits inside a WAV or AIFF file and what travels over S/PDIF. It is also the raw material that MP3, AAC and FLAC compress. PCM is not a format you pick; it is the one everything else starts from.