Ken,
Think of a PID as a "smart switch" It's a switch with some logical functions built in. I could go into a long/boring narrative, but WIKI does a pretty bang up job of explaining it.
http://en.wikipedia.org/wiki/PID_controller
A PID will basically automatically control your temps more accurately than a regular switch. I use one in my mash tun in a home brewery I built. The temp. control concepts are the same in both applications.
The SSR is short for Solid State Relay. The PID isn't able to switch the current to the heating element all by itself because of the wattage requirements of the heating elements, so the PID is wired to a relay and the relay turns on the element.
http://en.wikipedia.org/wiki/Solid_state_relay
The box pictured above is simply a fishing tackle box I had lying around. I think it'll work pretty well for this.
In the top picture, the left display is the PID. The top number is the current temp reading and the bottom number is the set/desired temp. Since the PID wasn't in operation, the bottom number was left at the default setting of 800. If I were using the PID to control temps, the setting would have been at my desired temp (225 for a butt). The left display (PID) is wired to this thermocouple:
http://www.auberins.com/index.php?main_page=product_info&cPath=3&products_id=101
The right display simply displays the internal temp of the meat. It's just the panel monitor I linked in the first reply. The panel display is connected to this thermocouple:
http://www.auberins.com/index.php?main_page=product_info&cPath=3&products_id=56
It seems pretty complicated, but it really is straight forward. I taught myself out of necessity. I was frustrated with the POS thermometers most folks used when homebrewing and started reading about the PID controllers. I'm just applying the same temp control concepts to smoking.
Any other questions, just ask.
cheers!