I will give you three answers that depend on your level of involvement in computers.
Mild: It talks and listens to the rest of the computer, to translate computer code into sound that people can understand. It can be used for many purposes such as play music, record your voice, voice recognition input with help of a mic. etc...
Hot: A sound card works by transfering and receiving digital and analog data as input/output device, that sits in the mother board to help processing of sound. It can be built in, external caled expansion card, or internal and sits on any of this busses isa/pci/usb.
Wild: I/O device, that uses An analog-to-digital converter (ADC)
A digital-to-analog converter (DAC) to convert sound waves.
A sound card must translate between sound waves and bits and bytes.
A very general answer would be:
Sound card listens to radio waves, )) ) ) )
and records the size of the waves.
Then creates bits and bites for the computer to understand.
Takes a sound wave sample and it splits it in sections.
this sections have diferent levels of sound.
For example a high level would be 10000, and a very low one would be 1.
it saves this values, in a process called SAMPLING RATE and it is measured in MHZ.
At this point the sound can stay in the computer untill you want to hear it again.
So you click play somewhere, and the computer inverts the process so you can understand the binary code.
One way it does it is by using FM synthesis by overlaping many of this recordings to create more complex sound waves such as the ones music and voice make.
And sorry for the spelling mistakes, the check spelling button was just hanging.