http://en.wikipedia.org/wiki/Sound_card
they take the audio off the cpu and process it separately, where they can do it with less noise/interference, so you get a cleaner sound. it depends how sensitive your hearing is really to which one you should get. and some onboards are really good now, i gave my x-fi away and just use my p6t-deluxe onboard as the difference in quality was negligible and the card was blocking half of my graphics fan intake. also i promised myself i wouldn't buy creative after the whole driver and support issues. so when i do get a new soundcard it will be an asus. shame really as i think the creative tech is better, but whats the point if you can't use it and get told they will sort it when they can - which then takes over 18 months.