Hi Pete,
You'll notice that no matter which radio you eventually choose, they all have at least three things (Also, "Hi" typically refers to a signal and "Lo" typically refers to a ground):
1) Mic Audio Hi - This is the sound from your microphone to the radio and what the radio eventually transmits over the antenna.
2) Comm Audio Hi - This is what you hear in your headset or what the radio receives - what you hear from the radio.
3) PTT / Mic Key - this is the Push To Talk, which engages the transmitter and allows said Mic Audio information to be transmitted.
Some will have varying degrees of additional info (if it's a Nav/Comm you'll likely also have a "Nav Audio Hi", or a terrain warning, etc..).
Typical radios will have between 3 to 10 wires dedicated to those things. They all eventually correspond to arms on the headset/mic jacks, either through an intercom, audio panel, etc.. (both internal and external).
Radios with internal intercoms will have similar outputs, but maybe more. If it's stereo, you'd have an audio L and audio R, etc..
Cheers,
Stein