Creative Sound Blaster Live!, a PCI sound card
A sound card (audio card) is the part of the computer hardware that controls the input and output of the sound signals. A sound card is what is known as an expansion card. This means that the card can be added to the motherboard.
Nowadays, most of the audio cards are integrated with the motherboard. That means they are built into a mainboard and can not be removed. In other words it is not an expansion card anymore. They only offer line-in, speaker, and microphone connections. For most users this is enough. When thinking of computer architecture, the integrated circuit sound cards occupy less space and thus became very popular and practical, especially for laptop computers. For more features like MIDI ports for connection of musical instruments and low latency requirements (so that the sound does not come distorted under heavy system usage) sound cards as expansion cards can be used. Modern expansion sound cards use the PCI computer bus standard. Earlier sound cards used the ISA computer bus, which was half-duplex, so the sound cards could not record and play simultaneously.