(The following information has been obtained from the USAjobs information on Veterans’ Preference – see the link above to the complete index of information available on the USA jobs web site).
Since the time of the Civil War, Veterans of the Armed Forces have been given some degree of preference in appointments to Federal jobs. Recognizing that sacrifices are made by those serving in the Armed Forces, Congress enacted laws to prevent veterans seeking Federal employment from being penalized because of the time spent in military service. By law, veterans who are disabled or who served on active duty in the Armed Forces during certain specified time periods or in military campaigns are entitled to preference over nonveterans both in hiring from competitive lists of eligibles and in retention during reductions in force.
Preference does not have as its goal the placement of a veteran in every vacant Federal job; this would be incompatible with the merit principle of public employment. Nor does it apply to promotions or other in-service actions. However, preference does provide a uniform method by which special consideration is given to qualified veterans seeking Federal employment.
Preference applies in hiring from civil service examinations, for most excepted service jobs, and when agencies make temporary appointments or use direct hire and delegated examining authorities from the U. S. Office of Personnel Management.