Thermo: "temperature"
Meter: "measure"
A thermometer measures the temperatures near it, but does not control them.

Thermo: "temperature"
Stat: "unchanging"

A thermostat controls the temperature of a device and keeps that temperature stable, by adjusting the amount of electricity that the device gets. The thermostat measures this with a temperature probe that you put in the enclosure or on your heat tape. You tell the thermostat what temperature you want, and it adjusts the heat up and down for you, so you never get too hot or too cold.

In your case, you would run it like this:

Flexwatt --->Thermostat--->Wall outlet.

If you don't use a thermostat on your flexwatt, it will get way too hot and can burn your snake. Flexwatt absolutely cannot be used without a thermostat. Good brands are Helix and Herpstat. I have Herpstats and haven't had a problem with them, but Helixes have an equally good reputation from what I've heard.

A rheostat is like a dimmer switch for your heat tape. You can turn it to a particular setting, and it will allow just that much current to flow through. This works okay in a room where the temperature never changes, but if the room temperature goes up and down a lot, the rheostat won't adjust the heat for you. It has no way to sense the changing temperatures, and just keeps putting the same current through no matter what.

Hope that helps a bit...