Kings of War describes North Korea's attempts to build a force designed for simultaneous guerilla and conventional conflict as a "hybrid" war. But some have described the simultaneous use of conventional and guerilla force as "compound war" (much as Lawrence and Allenby worked together to defeat the Turks in the First World War). This type of "hybrid war" differs from the hybrid war scenario put forth by David Kilcullen when discussing conflict in Iraq and Afghanistan. There, he refers to hybrid war in those regions as a simultaneous counter-narcotic/counter-insurgency/counter-terror/counter-sectartian conflict war.
Which leads me to believe that "hybrid war" really isn't a form of war. It's a way of looking at conflict. These forces--insurgency, terrorism, civil war, have always been a source of violence in any society. Our current vision of state-on-state war on an open battlefield is a relatively recent invention, dating back to the creation of nations around the time of Napoleon (First Generation Warfare). With nation-states gradually declining in their monopoly on the application of violence, it is only natural that state-on-state conflict will decline, giving rise to multiple forms of conflict in any given area.
That is the value of the concept of hybrid war--acknowledging the decline of state-on-state violence and the resurgence of these sources of small wars which have plagued great powers for thousands of years.