A sound card is a piece of computer hardware that allows audio playback.
Before the invention of the sound card, all computers could do was make beeping noises, and the aside from modulating the sound of the beep, no other noises could be made by the computer.
In the 1980s computer manufacturers started making expansion card hardware for PCs that allowed greater sound capability. These cards came with audio ports that allowed plugging in speakers or headphones in the same manner one would use a stereo or CD player, and would allow the playback of sound.
Recording of sound is also possible if the card is designed to allow usage of a microphone or a line-in connector, and proper audio capture software exists on the computer. Almost all versions of Windows come with the Sound Recorder built-in.
Modern motherboards have a basic sound card built in (usually Realtek), capable of playback and recording (through separate microphone and line-in inputs), which is sufficient for most users. However, high-quality sound cards with specialist features such as timecoding are installed as an additional internal plug-in card or are used externally through USB.