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.