Original: Some countries (such as the United States) use the General Officer ranks for both the army and the air force; others only use the General Officer ranks for the army, while in the air force they use Air Officers as the equivalent of General Officers. They use the air force rank of Air Chief Marshal as the equivalent of the specific army rank of General.
Simplified: A general officer is an officer who is at a high military rank, usually in the army, and in some places, the air force.