World War 1 was the first industrialized war in history. The war began as a war of movement, typical of 19th century warfare, cavalry skirmishes, open order infantry fighting, generals searching for the open flank. The war began its second phase in France with positional warfare. Armies dug into the earth resembled siege warfare on a large scale. The Western Front resembled the last years of the American Civil War and the Russo-Japanese War in the use of entrenchments.
The machinegun domuinated the battlefield and combined with heavy artillery shoots. This led to the reintroduction of body armor, tanks to break the trenches and support the infantry; aircraft used as a scouting platform, gradually became a tactical weapon with machinguns and bombs to isolate the battlfield.
Submarine raiders came into wide spread use to blockade England and Germany. This in turn led to the creation of sound detectors to track a submarine under the waters.
The last element that had no precedent in previous wars was the introduction of chemical warfare. Defenses against gas spurred research into more lethal means to dispense chemicals and ended with mustard gas, a blistering agent that attacked the enitre body, not just the lungs.
World War I gave a glimmer of implements of warfare taken for granted in 21st century warfare. Air support, mechanized warfare, submarines, radar contributed to the Allied victory in 1918. German officers refined the processes of these lessons and in 1939 unleashed the finished product that World War I began.
The conflict's name 'the first world war' gives you a major clue. WW1 was the first industrial, global war. It was fought using 'modern' industrial technology, transport and communications.
The war began with lancers on horses making cavalry-charges but it ended five years later with modern tank battles and aerial bombardments of towns and cities.
It was different to previous wars in many ways, but mostly it was the scale. It was 'total war'; the complete commitment of a country's economy and population to a military struggle. Factories on both sides could make vast amounts of modern equipment like machine guns and grenades, planes and even chemical weapons.
It also included more countries than any previous war and produced casualty rates on a scale never seen before (At the Battle of The Somme, on 1 July 1916, the British suffered 57,470 casualties, including 19,240 dead in just ONE DAY of a four month offensive) In total at least 15 million people were killed.
The human race had never seen anything like this war before.