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 non-veterans in federal hiring from competitive lists of eligible (or basically qualified) job candidates.
The Office of Personnel Management (OPM) website gives guidance on entitlement to veterans' preference in employment. OPM also overseas entitlement to veterans preference.
Veterans preference is governed by Title 5 of the United States Code, section 2108.