A combat engineer (also called field engineer, pioneer or sapper in many armies) is a soldier specialist who performs a variety of construction and demolition tasks under combat conditions.
The combat engineer's goals involve facilitating movement and support of friendly forces while impeding those of the enemy. Combat engineers build, repair and maintain buildings, roads and power supplies. They employ explosives for construction and demolition projects, and clear minefields using specialized vehicles. Such tasks typically include constructing and breaching trenches, tank traps and other fortifications, bunker construction, bridge and road construction or destruction, laying or clearing land mines, and other physical work in the battlefield.
Usually, a combat engineer is also trained as an infantryman, and combat engineering units often have a secondary role fighting as infantry. There are no advanced academic qualifications required to be a combat engineer. The term "engineer" is not to be confused with the term applied to Professional Engineer or Chartered Engineer.
Source: Wikipedia, "Combat engineer", available under the CC-BY-SA License.