Depends on what audio interface you're talking about, I have the E-MU 1616.
You have two XLR inputs for guitars and microphones, a headphone output with 1/4" jack and volume control, monitor output.
Eight out- and inputs 1/4", and of course the DSP processing emory on the interface, which is used by plugins you use in your DAW instead of your PC's memory and CPU, that way you can have a lot more plugins in your project before the PC is having a hard time.
Next to that there are so man more things, too many to talk about.