Short answers, pulled from the story.
Trench warfare is a type of land warfare using occupied lines made largely of military trenches, where combatants are well protected from small arms fire and substantially sheltered from artillery. It became archetypically associated with World War I, fought between 1914 and 1918, after the Race to the Sea rapidly expanded trench use on the Western Front starting in September 1914.
Trench warfare caused a stalemate because a revolution in firepower was not matched by similar advances in mobility, giving the defender the advantage. Continuous trench lines from the Swiss border to the North Sea coast of Belgium had no open flanks, so frontal assaults and their heavy casualties became inevitable, and the struggle settled into attrition.
A well-developed trench had to be at least 2.5 meters deep so men could walk upright and remain protected. British dugouts were usually 2.5 to 5 meters deep, while German dugouts went much deeper, typically a minimum of 12 feet and sometimes three stories down with concrete staircases.
The main killer in the trenches was artillery fire, responsible for around 75 percent of known casualties, followed by gunfire from rifles and machine guns. Disease was rampant, including trench fever spread by body lice, which infected over one million Allied soldiers, along with trench foot, gas gangrene, and infestations of trench rats.
The hand grenade and the machine gun became defining weapons of trench warfare. By the war's end 75 million British Mills bombs had been used, and the machine gun's defensive power was shown on the first day of the Battle of the Somme, when 60,000 British soldiers became casualties, mostly under machine gun fire.
The stalemate was broken through new tactics and machines. The Germans developed infiltration tactics using stormtroopers who bypassed strongpoints, while the British and French turned to tanks such as the Renault FT. The development of armoured warfare and combined arms tactics let static lines be bypassed, leading to the decline of trench warfare after the war.