From Wikipedia

Vietnam - Contact Ambush

Guerrilla warfare is the unconventional warfare and combat in which a small group of combatants use mobile tactics in the form of ambushes and raids to combat a larger and less mobile formal army.

The guerrilla army uses ambush and mobility in attacking vulnerable targets in enemy territory. Guerrilla warfare is countered with counter-insurgency warfare.

This term means "little war" in Spanish and was created during the Peninsular War. The concept acknowledges a conflict between armed civilians against a powerful nation state army, either foreign or domestic.

The tactics of guerrilla warfare were used successfully in the 20th century by, among others, the People's Liberation Army in the Chinese Civil War, the Irish Republican Army during the Irish War of Independence, and Fidel Castro's rebel army in the Cuban Revolution. Most factions of the Iraqi Insurgency and groups such as FARC are said to be engaged in some form of guerrilla warfare.