"Shell Shock" is the term describing the effect of war's brutality and pain on the otherwise-healthy human mind. Soldiers who signed up for war -- or in the case of World War I, were drafted -- were usually of sound mind before entering into service. After experiencing warfare up close, soldiers often had extreme stress reactions, ranging from mental breakdowns (amnesia, startle reactions) to violence against friends and non-injury physical ailments (tinnitus, sensory loss). During WWI, these symptoms became very common and are seen as a "classic" injury suffered during the war; ground troops were subjected to constant shelling from air and ground artillery and the symptoms were at first thought to be entirely physical, a response to the concussive and auditory effects of the shelling. Meanwhile, soldiers continued to suffer from shell shock effects, even those who were not close to bombed areas; diagnoses started to focus on the mental and emotional effects, rather than the physical effects.
These effects remained with soldiers even after the war, leading in many cases to post-traumatic stress disorder. Soldiers who returned from war, even if they weren't physically injured, were often unable to return to their original lives, and were referred to as the "Lost Generation." Because of this, the death-toll of WWI and the casualty-toll are significantly different; many more people were affected by the war than were directly killed by it.
First, please note that I have changed your question to refer to World War I rather than to World War II. The term “shell shock” is mainly applied to World War I and your other questions appear to refer to that war as well.
Shell shock added greatly to the casualties of World War I. It did not kill people or physically injure them. Even so, it did create casualties in that it made some men unable to fight. The BBC link below tells us that somewhere in the range of 80,000 British soldiers ended up being unable to fight because of “shell shock.”
Shell shock was essentially caused by the exposure of soldiers to too much trauma. This was partly trauma caused by the shelling that they often endured. Soldiers who were forced to undergo long artillery barrages could crack under the stress. However, it was also caused by other horrors of war. As can be seen in the BBC link below, it could come about when a soldier was forced to do or to see too many horrific things like (as in the case of Arthur Hubbard) killing Germans who were trying to surrender.
Thus, shell shock caused many casualties by stressing men so badly that they snapped emotionally and were no longer able to function well enough to fight.