An insurgent is a guerrilla fighter, usually an armed civilian.
Examples:
Boer fighters in the Boer war
Marquis and Partisans in WW2
Modern Iraqi/Afgani insurgents
Some kinds of American militia in the Revolutionary war.
Viet Cong in the Vietnam War
Insurgents are usually arm themselves when they feel their way of life is being encroached upon. Their quality and Morale range wildly. Some are zealous fighters and some empty a clip or fire off a few mortar rounds on their way home from work.