The term usually refers to the audio portion of the process although having MIDI I/O is good too. The MIDI allows you to control software instruments, record the patterns/note data in your MPC to a recording software app or also have MIDI tracks in a software app trigger the MPC.
You can record the output of the MPC into a computer's stereo 1/8" input from the stereo 1/4" outs of the MPC. You'd need to mute parts of your song to get multi tracks in the recording app so you would have maximum versatility to do a good mix in the computer to make your final product.
But the 1/8" input isn't very good quality, so you usually the audio version of the MIDI interface you have. MIDI is only trigger data not carrying the sound/audio itself.
Most audio interfaces have the MIDI too but if you wanted to buy one you could look at one without MIDI because you have the MIDISport already. But again it's rare that audio interfaces don't have both.
Do even 30 minutes of reading on this site and it will all be a lot clearer to you, and then any questions you need to ask here will be easier to formulate. Once you have the basics down with some reading you'll be a lot clearer on the generalities of what you're trying to do, and I guarantee already feel the progress you've made just off that, from there it just gets easier. But the basic terms and methods are a must to understand in the beginning or it will be headaches around every corner.
http://tweakheadz.com/guide-to-home-and ... c-studios/