Comparing Internal PCI Sound Card and External Sound Card

I bought a computer last year. I want to redesign the capabilities of this computer. I want to know the benefits of using external sound card? What do you mean by sound codec or audio codec? How much will it cost for a typical external sound card? Can you enlighten the difference between internal against external sound card how beneficial it is for the users? Will external give more functionality and better sound quality rather than the external? Please help. Thank you!
