Bombers are land-based aircraft. They can attack enemy land units (and sea units, but not very effectively), and they can weaken enemy city fortifications. Bombers cause collateral damage when attacking.
The bombing of strategic targets from the air dates back to the beginning of World War I, when the Germans used zeppelins to bomb Paris and London. Because zeppelins proved to be extremely vulnerable, they were withdrawn from war in favor of airplanes. By the end of World War I planes were being designed specifically as bombers.
The bomber's job is to carry bombs into enemy territory, normally beyond the range of artillery, and destroy targets of military and economic value. Bombers were often designed for specific missions, such as attacking ships, bombing vehicles and railroads, daylight precision bombing, and carpet-bombing.