From Citizendium, the Citizens' Compendium
Guerrilla warfare (also known as Guerilla warfare) is a set of strategic, operational, and tactical actions, within a political context, taken against an enemy in territory dominated by that enemy. Enemy forces are expected to be stronger than the guerrilla force, either because the area of operations is the enemy's home area, or an area that he occupies.
The term originated in the Peninsular War, from the Spanish word for war, guerrero; "guerrilla" is "little war". In usage, both "guerilla" and "guerrilla" are correct, but "guerrilla" is the more common of the two spellings and more closely follows the root word.
Since the enemy is stronger, the guerrilla force must fight only on terms favorable to the guerrilla; guerrilla warfare, although centuries old, is, in modern terms, asymmetrical warfare. In its ever-present political dimension, it is a form of insurgency.
"The enemy advances, we retreat; the enemy camps, we harass; the enemy tires, we attack; the enemy retreats, we pursue." — Mao ZedongGuerrillas frequently do not wear uniform and hide among sympathetic civilians, so they are may be in violation of lawful combatant status of the Third Geneva Convention. They claim military necessity for this approach, as they would be annihilated in conventional combat.