This is probably a dumb question but it's something that I've been curious about but have never really found out.

Out of all the pins on an OBDII connector only one is used for transmitting data, right?
I just don't understand how so much information can be carried on one single wire. If you're looking at a data list on a scan tool you can be viewing RPM, ECT, IAT, MAF, MAP, and a million other things all at once. How is all this data kept separate when it's just one wire?

And what exactly does the information transmitted "look like?" Like, if you hooked a DMM inline or something, what would it display?
I just want to understand what's physically happening when you're looking at a data list or flashing or anything like that.

The physical stuff is well described in the links above.

The single wire communication can be a bit annoying since transmit and receive ports are connected together.
Everything sent on the wire will also echo back to the one who sent it, so the software must be prepared to deal with these echoes.