Guerrilla warfare, also spelled guerilla warfare, is a type of combat that's fought by a civilian population or other people who aren't part of a typical miltary unit. Often, guerrilla warriors (often just referred to as guerrillas) are attempting to overthrow an existing government or are rebelling against a much larger, organized military (although in some cases, guerillas fight against rival insurgent forces).
What is guerrilla warfare?
Guerrilla combat often involves surprise attacks such as ambushes and raids, or sabotage of a vulnerable target. Many times, guerrilla warriors are fighting in their homeland or they have the support of the local population. Therefore, guerrillas are usually familiar with the terrain and landscape, and they use this to their advantage in their attacks: the enemy has no idea what's happening until the guerilla attack is underway.
As they are usually fighting against a larger, more fortified but less mobile military or police unit, guerrillas move in quickly and keep their battles short. By surprising their enemy and then retreating almost immediately, they keep their foes from adequately defending themselves or staging a counter-attack.
The word guerrillameans "little war" in Spanish. The term was first documented during one of the Napoleonic Wars (the Peninsular War of 1808-1814) when the British enlisted Spanish and Portuguese guerrillas to help them oust the French from the Iberian Peninsula.
A variety of other words mean pretty much the same thing as guerrilla, including rebel, insurgent, irregular, and partisan.