I want to invest in a thermostat/rheostat (not sure if there's a difference in what each entails?) but I wanted to know if there's limitation on what they can control. Can I use one to control a basking lamp? Under the tank heating pad (extending the question for my boa's enclosure here). I'm wondering because I don't imagine a thermostat would be able to control the heat output of a basking lamp but I might be wrong.
Thermostats automatically control heat (like it you set it to 110º in the basking spot, it will keep it to 110º at all times. Rheostat is just a dimmer, so it you want it at 110º, you'll have to adjust the slider until you get to 110º. I got my rheostat on Amazon for about $10, and it works great! I'm pretty sure thermostats would work on a heat pad, as well as the rheostat.
As jess mentions the big difference is what they do.
A rheostat controls voltage to a device, you can dim a lamp or a heat mat by limiting the power to it. A rheostat can't control temps if they get too hot for whatever reason, like the AC goes it, it just lets you adjust them.
A thermostat has a feedback system with a temperature sensor that works with a rheostat or similar component (dimming thermostat) or a switch (on/off thermostat). A stat can control temperatures to within a few degrees of a set point.
Rheostats are about $10, on/off thermostats about $30, dimming stats about $100
A dimming stat is fine for a lamp but a in/off would cause flashing if it hits the trigger temp.
So starting off (on a budget), a rheostat should be decent coupled with an accurate thermometer in the area of the lamp and regular monitoring of the basking area temp?