Mainly used by Britain and Germany, submarines were mainly used as stealthy war ships with a sole purpose of sinking enemy ships and/or submarines. First referred to as the "U-boat" (Undersea boat), submarines were used by the Germans first as tools of war to sink British ships using self-propelled torpedoes. The whole submarine was built in a torpedo shape for underwater dynamics and silent travel.
The submarine affected the world we live in today because of the possibilities that they would later open up for mankind. Underwater exploration as well as being able to map the ocean floor became possible because of the development of the U-boat/submarine.
The machine gun was invented to rapidly fire a flurry of bullets without having to reload. It was basically a stationary (tripod) assault rifle that was armed by 4-6 men and could fire 400-600 rounds per minute. They tended to overheat quite rapidly so they needed to be cooled with water, or air (which didn't come about until later on in the war).
The machine gun would go on to change warfare as we know it. The idea of having a larger amount of bullets in one clip would make it easier to chip down enemy forces. The tripod and long barrel allowed for more stability when firing, and this would go on to change the way guns were designed and built.
Its first use was by the French, but the main development of poison gas was done by the Germans. Poison gas first took form through tear gas, and later on through chlorine gas. led to tactics of first shrouding the battlefield in poison and then sending troops in armed with gas masks and guns to finish off the enemy.
Poison gas also changed the way that wars were fought and led to the development of more advanced weaponry. Smoke grenades and poison gas would be used in future combat and the use of chemicals against one's enemies would be played to their advantage.
Tanks in WWI came into play/development after countries started to notice the death toll that the long lasting battles of trench warfare were causing. First used by Great Britain and mass produced by France, the introduction of tanks would go on to assist the allies in many battles against Germany and turn the tides in their favor.
Tanks probably have had the biggest affect on warfare today out of the four. The combination of firepower, bullet resistance, and mobility that a tank possesses would go on to affect the builds of other machines of war such as helicopters, planes, and armored cars.