You need a midi interface if you're going to input trigger info from the MPC pads.
You need an audio interface if you're going to record audio from the MPC into Garageband.
Get an interface or card with both I/O options, if you want to do both of the above.
Midi is data, and will generally work between any devices so long as you have the inputs and outputs.
Basically, what Smartie said, just longer.