@JARblue the particular one I use is fixed temp. I think there's a thermocouple built into the tip, which is how the power supply can regulate the temperature of the tip. It doesn't really heat up until you put a load on the tip, so it doesn't end up overheating and deteriorating the tip.
It's pretty awesome, and I highly recommend it. With hobby use, these tips will last years. I'm still on my 2nd tip, and I still have brand new spares from 10 years ago. Cost is around $20, so it's very nice.
This is the one I have, but it has been replaced by newer (smaller) models:
http://www.surplus-electronics.co.uk....800.-1.85.jpg