"Consider a dental patient (the principal) wondering whether his dentist (the agent) is recommending expensive treatment because it is truly necessary for the patient's dental health, or because it will generate income for the dentist. The two parties have different interests and asymmetric information (the agent having more information), such that the principal cannot directly ensure that the agents are always acting in its (the principals') best interests."
http://en.wikipedia.org/wiki/Principal%E2%80%93agent_problem
"Consider a dental patient (the principal) wondering whether his dentist (the agent) is recommending expensive treatment because it is truly necessary for the patient's dental health, or because it will generate income for the dentist. The two parties have different interests and asymmetric information (the agent having more information), such that the principal cannot directly ensure that the agents are always acting in its (the principals') best interests."