This is very interesting. Care to share some basic information on these?

I have been using a couple pumps and aquarium heaters in my kitchen sink but I want to make something that is better suited.

So, if I get this correct, you bring A/C power to the PID controller, and wire a relay and sensor (thermocouple?) to it. You wire your heating element to the relay?

PID monitors sensor and applies the power to the relay when the sensor reaches below your set temperature, supplying power to the heating element?