Guerra is Spanish for war.
Guerrilla is small war.
As opposed to conventional, usually
larger, military units, guerrilla warfare is a bit more flexible.
You are fighting in small units, sometimes even all by yourself.
Who Came Up With the
Expression?
The term guerrilla caught on during the
Peninsular War, during which Spanish fighters drove
Napoleon crazy.
Your aim would be to separate the guerrillas from
one of their main assets — the general population. How to
do that? You shift the people into areas where they can be better
supervised and controlled, i.e. camps or "pacified" hamlets.
Anyone outside can thereafter be shot on
sight.
So it was done in Mexico,
see example here, in South Africa and Malaya by the British, and
in Vietnam by the Americans.
Guerrilla Trivia
La Guerrilla is also a 1972 movie, if you wanna brush up on your
Spanish.