Conflicts of interest and moral hazard issues that arise when a principal hires an agent to perform specific duties that are in the best interest of the principal but may be costly, or not in the best interests of the agent.
(Investopedea, at http://www.investopedia.com/terms/p/principal-agent-problem.asp#ixzz3bcFOgizG, accessed 30 May 2015.)
The principal-agent problem develops when a principal creates an environment in which an agent has incentives to align its interests with those of the principal, typically through incentives. Principals create incentives for the agent to act as the principal wants because the principal faces information asymmetry and risk with regards to whether the agent has effectively completed a contract.
See also the teaching topic, Agency Theory.