The most commonly used weapon in World War 1 was barbed wire. trenches reached up to 645 kilometres most of which was protected by barbed wire. Although bombs were being thrown around on the battle field, most of the barbed wire still remained intact and deadly to the opposition.
The main weapon used by soldiers in the trenches was the bolt action rifle. it could fire up to 15 rounds per second and up to 1,400 meters in distance. Mounted machine guns needed 4-6 men to fire them and had to sit on a stable flat surface. Mounted machine guns had the fire power of roughly 100 bolt action rifles.
On 1915 during the battle in Ypres the German army were the first country to use chlorine gas. Chlorine gas caused severe irritation in the throat followed up by heavy chest pain. However there was a big flaw with chlorine gas, it had to be the right weather. If the wind was too strong you would end up gassing your own troops.
Mustard gas was the deadliest gas used in World War 1. It would be held in a grenade like shell and thrown into trenches to kill off the opposition troops. The canister after detonation actually takes up to 12 hours to release the gas. the effects of mustard gas is blistering skin, vomiting, sore eyes, internal and external bleeding.
Tanks were introduced in World War 1 during the battle of the Somme. The were created to be able to remain stable in serious weathers. The first tank ever made was called "Little Willie" and needed at least 3 troops controlling it. the maximum speed for it was 3mph and could not cross over trenches.
Planes were also used for the first time throughout World War 1 however they weren't advanced at all. When they first started using planes they were used to drop bombs off and spy on other countries. As the war progressed planes were used more as fighter planes with mounted machine guns, bombs and occasionally cannons.