xkcd

In the U.S., they meter gas by the "therm," which is defined as 100,000 BTUs. It's a misconception that it's equal to 100 cubic feet of natural gas at standard temperature and pressure, and is merely a coincidence that those values are very close.

BTUs are like a shitty imperial calorie, the energy it takes to heat up one pound of water by one degree fahrenheit.

Also, don't confuse therms for thermies, a totally different unit that means the amount of energy required to heat up a tonne (1000 kg) (not to be confused with the imperial ton that is 2000 pounds) of water by 1°C.

Energy is so useful in so many different contexts that we can just always expect a million ways to express it.