As far as the fighting of World War I goes, there are probably 2 basic key concepts to take away: trench warfare & new technology. Because of new technology, soldiers built trenches, and because soldiers were hiding in trenches, new technology was developed for new tactics of fighting.
The fighting in WWI was in trenches. When the fighting began, Germany expected to be able to knock France out in a jiffy. But new technology made war more devastating and advances harder and more costly. Basically the lines of the Western Front (in eastern France) were formed in the first year of the war and they didn't move for the next 3 years. So, the armies built intricate trench systems. They used balloons and then, increasingly, airplanes to scout out enemy lines and take photos. They developed mortars which are like canons that are aimed at a high angle so they fall down into trenches. And, they developed gas, which came in different forms, some were odorless and invisible, that had terrible effects on skin, on lungs.
A very important thing to know about WWI was that it saw drastic technological advancements. Now, the American Civil War also saw great technological advancements that made it so bloody, but the leaders of Europe did not know much about it. And when they let tensions escalate so, they had no idea that it would produce such a horrific war. And when they preached national pride and honor in battle to their young men, they did not know that World War One would claim the lives of 9 million men, and leave many others physically and emotionally and mentally scarred.
But battles of World War One were horrific. A battle would take months, and claim the lives of a million men. In the Battle of Verdun, the battle tactic was attrition: to wear down the opponent, to kill as many as possible. One side would fire mortars and shells on the other side for days at a time - 7 days, 8 days. Meanwhile soldiers are dying, digging deeper, going crazy from the noise, from the vibration. Then, the offensive side would fire one huge mortar and there would be silence. Then troops would rise up out of their trenches and march across No Man's Land. They thought they'd find empty trenches, but instead they were hit by intense machine gun fire, and they fell fast. Those that reached the enemy trench faced men in gruesome hand-to-hand combat, killing men whom they had no argument with, who perhaps were peasant farmers just like them. And maybe the offensive side would gain 5 miles.
Fighting of World War One does have some fascinating stories. Dogfights are fights between two aircraft. They flew around in dizzying circles, firing at each other. It was all so unadvanced and the machine guns positioned on them (at least in earlier years) were immobile, so a pilot had to be aimed straight at his enemy to fire, in which case he was a likely target. Men in the trenches would watch dogfights overhead for entertainment. The famous Red Baron was a German fighter pilot who became known throughout Europe and even in America. He was not an ally, but British kids were still fascinated by him. He won some 20 or 30 battles, which is a record, before he was shot down in 1917. (And this is who Snoopy dreams of being...and you have perhaps heard of Red Baron pizza.)