A guerrilla is a civilian who attacks a regular army. When civilians attack civilians, it is usually called terrorism. When two or more armies (representing countries) attack each other, this is a war.
Guerrilla warfare is a war tactic in which people (Spanish: guerrilleros) fight against an organized army. Guerrilla warfare is sometimes practiced in places where a regular army has difficulty, such as forests and mountains. Usually, this army is invading a territory. In open fields, the organized army, which is better armed and larger, has the advantage. But in forests and mountains, the guerillas can gain an advantage over even large and better armed armies.
Guerrilla is a word of Spanish origin. It means "little war". It was first used in 1808, when Spain was invaded by Napoleon, resulting in the Peninsular War with Spanish guerilla forces resisting the French Army.
This war tactic was used on the British by the Native Americans. The natives had an advantage because they used guerrilla warfare against the British Army. During the American Revolutionary War, the Americans used guerilla warfare against the larger and better armed British Army. Guerilla warfare was commonplace in the Border states (American Civil War). This war tactic was also used on the Americans during the Vietnam War.