Where is this and do I just turn it off to stop water going into my heater core?Is this something I do in the summer to make my AC work better?I have an 82

Should be in the water hose going to the heater core. It is vaccuum controlled. Look on the pass. side, down between the exhaust and the fenderwell area, and you should be able to see the control valve, spliced into the hose.
Sometimes the valve will get stuck in a particular position, and not function properly.
The only way to totally remove the heat generated by the hot water in the hoses is to tie the two hoses together, coming from the engine. Just blocking the flow thru the core with a "shut-off" valve won't completely eliminate the hot water from reaching the core. If you tie the hoses together, you eliminate the hot water from being in the heater core at all, therefore eliminating the source of heat.

Joel,

What I mean by "tying" the hoses together is, take the hoses loose from the heater core, and simply splice them together. Another way to accomplish the same objective is to remove both of the heater hoses from the engine end, and plugging them there. This is better, as it keeps from possibly damaging the heater core tubes by pulling/twisting on the hoses, trying to get them off.

A shut-off valve will keep the hot water from flowing freely thru the heater core, but it does not keep the hot water from touching the core, thereby transfering heat to the core. As long as the hoses are still connected to a hot water source(engine), and the core, there will be tranferance of heat.(conduction)

This method will work on any vehicle. All heater systems will have water flowing in one side, and out the other. Stopping the flow on one of the hoses will help some, but as long as the hoses are connected to the core, and have hot water in them, the heat will transfer to the core. Completely removing the hoses from the core will result in a 50deg drop(avg.,from ambient) in the air outlet temps, on a 100deg day!
This doesn't mean the A/C temp will be 50deg. cooler, just that the air temp difference from outside(ambient) to the inside air outlets will be much cooler, as the A/C doesn't have to cool the heated water in the heater core. A/C air flows over the heater core before it gets to the evaporator, in most systems. In our Corvettes, there is a flapper door that directs the outside air from the blower motor either thru the heat side, or the A/C side. As long as there is hot water in the heater core, the air temp will be higher.

As a side note, you can replace your heater control valve with one from a later model 'Vette('84-'86 or so), and it will control the flow of hot water much better. They seal better, and last longer to boot.Adams' Apple2006-12-25 21:57:02

Will it make my AC run a little cooler?

yup!

Are these things supposed to be turned off in the spring and turned back on the in fall.Just what are they for I guess.I've never had a car with one before

The factory valve is vaccuum actuated. It should be off with the A/C on "Max" only. Any other setting will allow some hot water flow.
You can replace the factory valve with a "manual shut-off" valve, and turn it off, but the heat will still be able to get to the core. The only way to completely eliminate the hot water from the heater core is to disconnect the hoses.
If your car's A/C is not cooling well, you may have other issues, also, and not just hot water in the core.
For other folks, if you have a car that doesn't have A/C, disconnecting the heater hoses will absolutely lower the interior temp in the summer.

My AC works fine,just had a bunch of stuff done to it.blows cold but I can still feel some heat down by my feet.

Welcome to Corvette Land!
You can install some heat barrier under the carpet, and it will help a bunch. Also, make sure there are no open holes in the firewall that will allow hot air to enter. This is a big problem.