How does MIDI distinguish between simultaneous and consecutive notes?

I'm trying to make a software that can properly interpret MIDI data and play it (and I have no idea what I'm doing). Everything I've looked at so far seems to show that there's no way to tell whether notes are supposed to be played at the same time or one after another, but all MIDI players figured that out SOMEHOW. Really I just want to know how to parse MIDI data. Are there any good open source scripts (in any language) designed for this task I could look at to use as a reference?