World War I, at the time referred to as The Great War or The European War, lasted from 1914 to 1918.
When Archduke Franz Ferdinand was shot on June 4, 1914, Austria sent Serbia an ultimatum demanding concessions that they didn't expect to get, hoping to provoke a war. Serbia sent back their answer a mere two minutes before the deadline, and did not make clear what their answer was, but it was not unfriendly. Austria attacked anyway. Russia had promised Serbia support in case of an attack. France had assured Russia of its support. Great Britain was hoping to reach a peaceful solution, and Germany had told everyone to stay out of the whole thing.
It ended up with Germany declaring war on Russia on August 1, and then on France on August 3. En route to France, Germany attacked Belgium. Belgium was on Great Britain's side, so Great Britain declared war on Germany on August 4. Things escalated from there. The U.S. was mostly trying to get everyone to stop fighting, but the Germans kept sinking their boats; then the U.S. found out that the Germans were trying to get Mexico to attack them, and the U.S. joined the Allies.
10 million died, 20 million were wounded. The countries involved spent an estimated $200 billion trying to kill people. This was, as stated, the great war. Partially, this was due to the number of participants, but it was also the first European war in which machine guns, submarines, torpedoes, tanks, airplanes, and chemical weapons were used in large scale, and these new technologies changed the war, making it unbalanced and unpredictable in surprising ways.
Machine guns and advanced artillery made infantry much less effective; well armed ground troops on both sides found that they were canon fodder, and the only way to combat this was to dig trenches, and stay in them. Trenches were wonderfully effective, and entrenched troops could hold positions almost indefinitely, which did not stop the higher-ups from trying anything they could to advance on the enemy. One common tactic was attempting to flank the enemy, digging the trenches laterally until they were able to sneak around and attack from behind; this was easily countered by the enemy by the simple expedient of likewise keeping pace in extending their own trenches. This led to ridiculous stagnation, as when the 1914 Race to the Sea resulted in the French and German armies building flanking earthworks in a continuous 400-mile line extending from Brasles northward until it eventually dead-ended at the North Sea.
The Germans were to come up with an unpleasant solution to the (in)effectiveness of trench warfare: chemical weapons. On April 22, 1915 the Germans released chlorine gas over the French troops at Ypres, quite effectively breaking their line. The German army, however, had very little faith in the gas attack, and had not planned a concentrated advance through the broken line; by the next day, the Allies had already identified the nature of the gas and were putting protections in place (starting with simply putting a damp cloth over one's mouth; it worked, barely). Chemical warfare continued on both sides, causing great human suffering but very limited military success.
The other solution to the machine gun/entrenchment problem was tanks. The development of the internal combustion engine meant that not only could carts now move themselves, it meant that even very heavy carts could move themselves. Continuous track vehicles had been commercially available since 1901 (initially used by steam-powered tractors), and at the start of the war gasoline powered, treaded Holt tractors were being used by both sides to haul artillery around. The British quickly started work on a tank that could be used offensively; the Mark I entered service in August 1916. The French followed suit shortly, in April 1917, while the Germans, only really starting development once they saw the Allied tanks on the field, only managed to get 20 tanks deployed before the whole thing ended. The Germans did, however, lead the way in anti-tank weapons. By 1917-1918 this new vehicle had proven that it could advance the front line even against an entrenched enemy, and it was quickly on its way to becoming the new cavalry.
As an aside, the cavalry were still around; arguably made useless long ago with the development of the deadly Minie ball, the machine gun pretty much put paid to whatever value remained. Regardless, the military commanders liked tradition, so cavalry regiments sat in the background, mostly waiting for opportunities to charge that never came.
Meanwhile, the naval situation was comparatively peaceful. Both sides had invested, for decades, in an arms race to make the biggest boats with the biggest guns. Additionally, in 1906 the HMS Dreadnought had introduced steam turbine engines to the world of battleship design (previously, ships had used heavier reciprocating steam engines), which would result in much higher speeds. Meanwhile, while the English and Americans were proudly reducing armor to maximize their new-found speed boost, the Germans had invested in submarines, mines, and torpedoes. This made it much too dangerous for aggressors to enter protected German bays. In response, the British retreated to a 'distant blockade' of the Germans, hanging around at the furthest point that could be considered militarily useful. And they sat.
In February of 1915, Germany finally broke the taboo against using submarines to hunt merchant vessels, and started attacking British non-combatant ships. This worked very well for them from a military perspective, but the sinking of the Lusitania and then the Arabic resulted in enough American deaths to get the US angry, and the Germans.... stopped. They would start hunting merchant vessels again in February of 1917, but this had given the Allies time to prepare, and a working hydrophone and effective depth charges were in service before the end of the year, while Allied minefields had been placed to protect against German subs approaching England. Compared to ground troops, comparatively few sailors died, and the main outcome was the slowly dawning realization that dreadnoughts and the equivalent were not particularly functional in an actual war, and that the last 50 years of battleship design had been, in retrospect, a bit silly.
In contrast, the sky becoming busy. While zeppelins never fulfilled their projected potential, airplanes turned out to be pretty cool. The invention of a mechanical synchronizer allowed a propeller plane to shoot between the blades of its spinning propeller, making dogfights and strafing runs feasible, and bombers were used by both sides. The Germans broke another norm -- this one, arguably, forbidden by the The Hague Convention of 1899 -- when it started aerial bombing. They used first zeppelins, and then later Gothas and Giants (confusingly, the Giants, while definitely airplanes, are properly known as Zeppelin-Staakens) to bomb civilian targets, including London.
Aerial defense was quickly developed, and the basic technologies developed here would remain essentially unchanged up to WWII (and WWII's biggest improvement would be the invention of radar). The idea of paratroopers and airdrops were trialed, but barely used; the pioneering work done now would eventually be useful in WWII.
While not a new technology in the same way as some of the other innovations listed, the railway also deserves a mention. With the advent of widespread railways across Europe, troop movement became quicker and easier by orders of magnitude. Troops could be deployed in a matter of hours rather than days, and instead of being worn out by a long march, they would be well rested and ready to start digging some trenches.
A number of other technologies appeared around this time, and were useful in the war effort; many inventions owe their existence or their popularity to the war. These include air control (supported by advances in radio), daylight saving time, the flame thrower, tracer bullets, the aircraft carrier, the mobile x-ray unit, stainless steel, and cellucotton bandages. Of these, radio stands out as a major beneficiary of the war, although during the war itself a significant portion of communication was still done by telegraph wires rolled out across the fields.
All in all, it was a bad deal for humanity, and no one wanted to do it again. But they would.