A sound card is a hardware component in computers that enables the input and output of audio signals. It converts digital audio data into analog signals that can be played through speakers or headphones, and vice versa for recording. Sound cards can be built into the motherboard or installed as separate expansion cards. Most sound cards support various audio formats and can enhance sound quality for music, games, and movies. They often include features like multiple audio channels, surround sound support, and connections for microphones and instruments, making them essential for audio production and gaming experiences.