Total war refers to a state of conflict in which the entire population is mobilized to support the war effort. As such, there is little to no distinction made between civilians and combatants.
The first instance of this was during the French Revolution and Napoleonic Wars. The republic of France saw itself under siege by the entire collective strength of Europe. They mobilized more than a million men to defend their borders, the first time a force this size was mustered in European history.
The American Civil War was another important event because it saw widepspread campaignes of terror by the Union Army on the people of the south. General Sherman used his army to try and destory the ability of the south to fight. Towns, railroads, supplies, farms, and plantations were all destroyed.
World War II was another major moment in total war history as entire populations were targeted for extermination. The holocaust and treatment of civilian POW's were prime examples of this concept.