sound cards do the same thing audio interfaces do.
a sound card just means a card that plugs into your PCI port or is built onto your mother board.
Audio interfaces is an EXTERNAL soundcard that can plug into your comptuer via firewire, USB, etc.
they all do the same thing. They take analog and convert it to digital or vice versa.
there are all types of sound cards or audio interfaces. External ones, internal ones, ones that accept XLR inputs, ones that accep only RCA inputs, ones that only accept DB25 inputs, etc. There are high end fancy ones and cheaply made ones that sound terrible. It all depends on how much money you have and what your needs are.