A buddy of mine is "into" such things.
They're usually just "USB soundcards", in an external box.

I don't know if they sound better than the internal soundcard or not, but the primary benefit they may have is that they're outside the main box, and therefore farther away from any electronic noise inside the PC.

As they also often integrate a power-amplifier, there is less potential for noise entering the stream between DAC, pre-amp, and power amp.

Cheers