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Put succinctly, an actuary measures the financial implications of risk. There are many, many sites that do a good job of explaining the profession and general tasks expected of actuaries, such as:
http://en.wikipedia.org/wiki/Actuary
http://www.bls.gov/oco/ocos041.htm
Also, within the actuarial profession, there's insurance and consulting, in which fall pension, health, life, and property & casualty actuaries.