Guerrillas establish bases in what are called foca (singular foco) or base areas, with the theory being that these areas will gradually expand until the guerrillas control the entire countryside and the enemy is isolated in the cities. This is supposed to culminate in an eventual direct war, as happened in China. But today, guerrilla tactics are more often used to harass and impose costs on powerful armies so that they are unable to control a territory and are eventually forced to leave.
Guerrillas are archetypically left-wing, usually adhering to some version of Maoism, Guevarism or some other version of statist communism. It was from such currents that the idea of guerrilla war emerged. However, the term can also be applied to indigenous movements (e.g. the OPM), fundamentalists (e.g. the Afghan mujahideen) and even some right-wing populist groups backed by the US (such as Renamo in Mozambique). Although the emphasis on support from the impoverished masses gives guerrilla strategy a left-wing bent, it is a strategic approach and not a political tendency, so in theory a guerrilla can have any political perspective compatible with attempting to win popular support.
Iraqi insurgents have insufficient resources to confront American troops head-on, so they have resorted to hit-and-run guerrilla tactics.
Essentially guerrilla warfare is fighting an offensive war while staying on the defensive. The enemy is always on the offensive so a guerrilla force must always be on the defensive. The idea is to inflict as much damage as possible on the enemy by surprise and then return to a defensive position fast enough so that the enemy cannot find anything to retaliate against. This is done repeatidly to undermine the enemy until the guerrilla force grows stronger than the enemy at which point conventional war can be waged.