WWI aircraft were originally used for scouting, not combat per se. Of course, were there is war, machines are turned to such purpose, so they started carrying guns and carrying (yes, carrying, inside the craft) bombs, which literally were tossed overboard. More of a harassing technique to throw a bomb overboard.
In time, of course, on-board guns were exchanged for built in weapons - which presented problems...like when your machine gun that is literally mounted in front of the pilot (so he can pull the tigger) shoots off the blades of your aircraft, you tend to crash...thus engineers went to work, end developed guns that would fire in time with the propellor. Likewise, on-board-throw-overboard bombs were replaced by a bomb that could be carried and released...thus allowing for larger bombs...such that by the end of the war, aircraft had transformed from simple tools of reconnaissance to the precursors of the death machines they became in WWII. But as far as a destructive component of WWI, the aircraft was never truly that destructive as compared to the existing weapons on the ground - i.e. artillery and chemical weapons, which were far more devastating that aircraft.
Aircraft were also used to shoot down balloons, which were used for field recon -- balloons: the precursor to satellites, in a way.
There were a few famous WWI pilots - "The Red Baron", who really was a German Baron (I believe) Baron von Rictoff (speling is wrong - look it up) who had many kills from his tri-plane before finally disappearing/being killed - but by far the most famous pilot of WWI
Why tri-plane? or Bi-planes in WWI? Better lift, as the engines were not that powerful.
Hope that ll helps - just stuff off the top of my head.