Sound cards allow the computer to output sound. The quality of the card can affect directly the quality of sound you hear when listening to music either by speakers or earphones. the two main ports on a sound card is a sound out port which allows you to plug in speakers, headphones or earphones, finally a microphone port which basically allows you to input sound into your computer.
Today many motherboards come with in-built sound cards that usually cannot be removed but only disabled. Though cards are still more popular and can cost anything up to £500.
Sound cars usually feature a analog-to-digital converter this converts the input of digital data (sound) into a analog output, this output is then connected to a sound system of some kind. Some more advanced cards will include multiple sound cards to improve data transfer rate.
Most sound cards also have a "microphone" port, this allows you to connect a microphone for sound input for things such as VoIP.