I think meangreen's right, you are out of your element - but that's how you learn things. An engine fire can be a very educational experience!
I have a basic understanding of this, but I'm sure someone will correct me if I mis-speak.
Current and voltage are 2 different things.
Voltage is like electrical pressure, measured in volts.
Current is the actual flow of electrons, measured in amps.
The little idiot light on your dash is 12 volts, just like your headlights, but the headlights use much more current - to produce more light.
These numbers aren't right, but say your efan uses 30 amps. That's while it's running. It probably takes twice that to get it started and up to speed. That fan is going to try and suck that much current through your circuit, so you need wires and a switch that will move 60 amps without overheating and burning up. The shorter those wires are, the less resistance, less heat they build up. That's why you want to use a relay. It keeps your high amp wiring to a minimum and lets you buy other components (like the temp switch) that are rated for a lower amperage.
A relay is just an electrically operated switch. You put your relay (rated for the max current that you expect) in your high amp circuit, then run a smaller wire (because it doesn't take much current to operate the relay) to the temp switch and/or manual switch. Here
is a circuit diagram for a two speed efan setup with two temp sensors and two relays. If you google "efan circuit diagram" you'll find a lot more.