A typical sound card includes a digital-analog converter that converts recorded or generated digital waveforms of sound into an analog format. This signal is led to a (earphone-type) connector where a cable to an amplifier or similar sound destination can be plugged in.
Also, a sound card has a "line in" connector where the sound signal from a cassette tape recorder or similar sound source can be connected to. The sound card can digitize this signal and store it (controlled by the corresponding computer software) on the computer's hard disk.
The third external connector a typical sound card has, is used to connect a microphone directly. Its sound can be recorded to hard disk or otherwise processed (for example, by speech recognition software or for Voice over IP).
One of the first manufacturers of sound cards was AdLib. This set the standard until Creative Labs produced the Sound Blaster.
Early soundcards could not record and play simultaneously. Most soundcards are now full-duplex.
To use a sound card, a certain operating system typically requires a specific device driver.
wikipedia.org dumped 2003-03-17 with terodump