He may not know what a DAC is, nor any of the features that help make sound cards worth their while.
A sound card, like MEgamer said, is a dedicated DAC (digital to analog converter). It is what makes it possible for us to play digital media through our analog speakers. It converts digital IO signals and converts then to analog sound frequencies that our speakers can play.
It also converts analog audio to digital through recording software if the sound card in question has an input as well as the standard output.
Now the idea of a dedicated sound card is different than an integrated sound card because the latter is considered a soldered on part of the motherboard in the computer as opposed to the prior (dedicated) which is an add-in (PCI or PCI-E) card installed into the computer
The purpose of a dedicated sound card is to accelerate audio to a greater extent than an integrated sound card or "chipset" because of its more robust capabilities.
Some of those include video game and movie digital audio positioning and effects, better noise to sound ratio, more channels (for up to 7.1 speaker sets) and overall higher quality sound from your computer (some cards rival decent home theater amps).
Hope this helps, there is a lot more on wikipedia and various other sites including this forum for more information.