sci.energy

Wikipedia isn't bad on the use of "therms":
/wiki/Water_heater


For measurement units, it says:
Natural gas in the . is measured in CCF (100 cubic feet), which is
converted to a standardized heat content unit called the therm, equal to
100,000 British thermal units. A BTU is the energy required to raise one
pound of water by one degree Fahrenheit. A . gallon of water weighs
pounds. So, to raise a 40-gallon tank of 55 °F water up to 105 °F would
require 40 x x (105 − 55) / 100,000 BTU, or approximately CCF, at
100% efficiency. A 40,000 BTU (per hour) heater would take 25 minutes to do
this, at 100% efficiency. At $1 per therm, the cost of the gas would be
about 17 cents.

As for usage ... it goes on to say:
Water enters residences in the US at about 10 °C (50 °F) (varies with
latitude and season). Adults generally prefer shower temperatures of 40–49
°C (105–120 °F), requiring the water temperature to be raised about 30 °C
(55 °F) or more, if the hot water is later mixed with cold water. The
Uniform Plumbing Code reference shower flow rate is gpm (gallons per
minute); sink and dishwasher usages range from 1–3 gpm.