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) use the land and geography as an advantage against an organized army. Guerrilla warfare is sometimes practiced from dense areas such as forests, or jungles. Usually, this army is invading a territory.
Guerrilla is a word of Spanish origin. It means "little war". This war tactic was used on the British by the Native Americans. The natives had an advantage because they used Guerrilla warfare. This war tactic was also used on the Americans during the Vietnamese War.