What does "Guerrilla Warfare" mean?

Guerrilla warfare is war that is done in ways that are
different from how large "regular" armies conduct warfare.  Guerrilla warfare is often
conducted by informal groups and typically uses tactics other than the large, pitched
battles that are common in wars between major regular
armies.

An example of guerrilla warfare can be seen in the
Vietnam War.  Much of the conflict in that war involved irregular forces (often mixes of
regular soldiers from North Vietnam and armed civilian Vietcong from South Vietnam)
fighting in unconventional ways.  These tactics included small ambushes of American
forces or hit and run attacks on bases.  The point of this sort of guerrilla war is to
hurt the larger army a little bit at a time.

Guerrilla
groups are typically not strong enough to fight in conventional ways.  Therefore, they
resort to unconventional tactics like the ones mentioned above or like the IEDs that
were used so much against US forces in Iraq.