Guerrilla Warfare, pronounced gorilla warfare, is a type of warfare fought in a disorganized, hunter-like style. In contrast, conventional warfare is usually fought in organized regiments with artillery and support weapons.
Guerrilla warfare troops in MnB are all infantry units. However, typically, this excludes engineers, which are used for building structures, not considered Guerrilla warfare. On a more positive note, signalers and support are currently acceptable as guerrilla warfare.
There is a nifty medal for those commanders which can successfully exploit Guerrilla warfare up to wave 100. Keep in mind, that, usually, to achieve this ribbon, you must keep your total soldier count under 6, because German Support is not friendly.
Now, a commonly mistaken thing: Most people think that, because of the pronunciation, guerrilla warfare is jokingly, fought by gorillas. This is NOT true. It comes from the Spanish root guerra, meaning war and the suffix -illa, meaning little. It truly means "little war".
In real life, Guerrilla warfare is rarely fought by organized armies such as in MnB, and is rather fought by militia or civilian groups. It is most profitable in defensive positions where a local militia has knowledge of the terrain over a disadvantaged adversary. These groups exploit unconventional tactics when fighting, most often.