MIDI, Musical Instrument Digital Interface. A protocol for transmitting info between MIDI devices and or devices with some kind of MIDI support. Most often used in conjunction with a sequencer, hardware or soft. Think of it as a modern day player piano. You record the MIDI infor and the sequencer plays it back through what ever device you tell it too. Of course there are many other uses for MIDI. Sysex, footswitches, lighing control, but its all the same thing, information. MIDI is not audio, I repeat MIDI is not audio.
S/PDIF, Sony/Phillip Digital Interface, a protocol for transmitting stereo digital audio. Commonly found in the form of an unbalanced 75 ohm coaxial cable with RCA connectors, also can be transmitted optically. Considered by most to be "consumer" as aposed to "profesional" AES/EBU which is basically the same signal but sent on a balanced line with XLR connectors, though sometimes BNC's are used too.
1/8" jack is simply an analog means to transmit stereo audio, or balanced mono audio, but not in the case of your PC. It is not prefered, nor recomended for serious work. Generally the noise floor is very high, frequency responce is poor and the analog to digital conversion is the cheapest possible.