I have 73' camaros rs. The starter solenoid has 3 terminals on it. Batt, s , and r.. What do the 2 letters stand for? R? S? Also which terminals would I hook up a remote starter switch to?

The "S" terminal takes the purple wire, which energizes the solenoid/starter when you turn the key to "Start". The "R" terminal takes the other small wire (don't know what color it is on a '73), which feeds a full 12 volts to the coil (+) when the starter is cranking. You connect a remote starter switch to the big battery cable stud and to the "S" terminal.
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The "R" is for the Resistor wire bypass which as John said, sends a full 12V to the primary side of the ignition coil during cranking for "points style" ignitions. It is supposed to help boost the ignition for starting but it will usualy start fine even without the wire.

-Mark

Thank you for such quick responses.