What is it?
Trench warfare is a form of land warfare using occupied fighting lines consisting largely of trenches, in which troops are significantly protected from the enemy's fire and are substantially sheltered from artillery.
No Man's Land
This here is a photo of what is known as No Man's Land. In WWI, both sides constructed elaborate trench and dugout systems opposing each other along a front, protected from assault by barbed wire. The area between opposing trench lines (known as "no man's land") was fully exposed to artillery fire from both sides. Attacks, even if successful, often sustained severe casualties as a matter of course.