A sapper was a combat engineer, a combatant soldier who performed a wide variety of combat engineering duties; typically including, but not limited to, bridge-building, laying or clearing minefields, demolitions, field defenses, general construction and building, as well as road construction and repair. He was also trained to serve as an infantryman when needed in both defensive and offensive operations and was fully involved in modern counter-insurgency operations.
In the Malazan Military, sappers (also called saboteurs), particularly those of the Bridgeburners, were known for their knowledge of Moranth munitions. Since these munitions were originally meant to be dropped from the back of Quorls, the Malazans had to devise ways of adapting them so they could be used from the ground. See the linked articles on Moranth munitions and Crossbows for more detailed information on these techniques.
The Malazan army used to have specialist Sapper companies until one such unit, under the command of Captain Spindle, blew itself up when one of the sappers stumbled onto his own cusser. At that point, Dassem Ultor broke up the sapper companies.