The word guerilla is another way to spell "guerrilla": a member of a small, loosely organized army that fights a larger, stronger force. If you're a guerilla, you confront enemies unexpectedly and use tactics such as sabotage, raids, and ambushes.
A guerilla is not a member of a state-sponsored army. Instead he or she is part of a smaller, independent army that fights a larger, more established force by using tactics that include the element of surprise. Also known as an insurgent or irregular, a guerilla usually exists outside mainstream society and typically has taken a position of active revolt or rebellion against the established ruling power.