Wikipedia isn't bad on the use of "therms":
http://en.wikipedia.org/wiki/Water_heater


For measurement units, it says:
Natural gas in the U.S. 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 U.S. gallon of water weighs 8.3
pounds. So, to raise a 40-gallon tank of 55 °F water up to 105 °F would
require 40 x 8.3 x (105 ˆ’ 55) / 100,000 BTU, or approximately 0.17 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 2.5 gpm (gallons per
minute); sink and dishwasher usages range from 1€“3 gpm.