A ground attack aircraft is an aircraft that is designed to operate very close to the ground, supporting infantry and tanks directly in battle. They are used essentially as mobile machine guns and anti-tank guns against single targets, as opposed to bombers which typically attack more "strategic" targets. This classification goes by a number of names, including attack aircraft, fighter-bomber, tactical fighter and even includes the dive bomber.
Very few aircraft have been dedicated to the ground attack role, most that are used in this role are actually fighterss or light bombers. Most of the dedicated designs came from early World War II when the available power from aircraft engines was so limited that every plane had to be dedicated to a single task. The most successful ground attack aircraft would generally be credited to the Ilyushin Il-2 Sturmovik, which was credited by Stalin for winning the war. The Luftwaffe fielded a very similar plane, the Henschel Hs 129, but produced very few of them and they had no effect on the war effort.
By the end of that war the average day fighter had more than enough capability to carry out the ground attack role, and some of the most successful designs were slight modifications of the Focke-Wulf Fw 190 and unmodified Hawker Typhoons.
In the post-war era, relatively few aircraft have been dedicated to ground attack; examples include the A-6 Intruder, A-7 Corsair II, and Sukhoi Su-17. In the late 1960s the US Air Force designed the Republic A-10 Thunderbolt II in order to have a rapid and long-range way to disable enemy tanks. However this role is generally filled by US Army helicopters. The Soviets built the seemingly similar Su-25 Frogfoot, but this plane is used more as flying artillery than in the dedicated anti-tank role.
Nevertheless, the role remains well-defined and in use, resulting in dual designations like F/A-18 Hornet. More recently, the term strike fighter has been gaining currency as the way to refer to these dual-role aircraft.