What are facts about the trench warfare in world war one?

Trenches were dug by both sides in the war.

Trenches were used to protect men from artillery and machine-gun fire.

Trenches became death traps if the artillery could accurately locate them.

Trenches were also death traps after the introduction of poison gas attacks because the heavier than air poison would sink into the trench.

Trenches were sometimes very primitive ditches and sometimes were very elaborate with telephones, kitchen, latrine, field hospital, commissary, bunks, ammunition dumps, etc.

Trenches were sometimes haphazard and were sometimes extremely orderly, with a front line, a secondary line, and additional trenches for reserves and artillery.

Trenches were sometimes dug by one side and later taken and used by the other side.

Attacks from the trenches were called "over the tops" -- the men would climb out of the trench and over a small berm with firing positions into "no man's land". There they were exposed to withering enemy machine gun, rifle and mortar fire. Typically they would fix bayonets before attacking and depending on their weapon could generally fire only one shot (or none) while attacking.

Very near the end of the war, the Germans developed an effective attack strategy against trenches. Rather than use rifles with bayonets, they gave their attackers much lighter carbines that could fire many times before reloading. They also gave their men hand grenades and flame throwers in some units. These men could attack in lightning fashion in small units and often worked at night. These attacks were much more survivable (for the attackers) than the massed over the top attack.

Other strategies effective against trenches were precision artillery and / or mortars; aircraft with machine guns; zeppelin attacks; and attacks from the far ends of the trenches, often with tanks.