The Great War was the war that defined warfare for probably a very long time. Armies, unused to light infantry battles and the use of tactics over numbers, found themselves in a stalemate, "dug in" at opposing trenches that stretched for miles and miles. This warfare was particularly gruesome, as the cramped and dangerous conditions led to many serious diseases, both mental and physical. The phrase "over the top" was created in this war, when frustrated army commanders ordered their soldiers to simply get out of their trenches and charge the enemy positions. When units went "over the top," it was common for the entire unit to be completely wiped out, resulting in the deaths of thousands in a few hours.
The war itself was brought about due to rising tensions in Europe and the web of alliances that existed. When the Austrian Archduke Ferdinand was assassinated by Serbian terrorists, Austria blamed Russia, as Russia had always promised to protect Serbia should Austria look to seize it. Following this outbreak, Germany and Italy declared war on Russia's ally France, whereupon Britain was inevitably thrown into the fray.
Rememberance Day parades are held every year in every town in most of Europe (not sure about the U.S?) to remember the veterans who fought in it. Hundreds of local military personnel attend the parade and there is a Church service with the famous words being read (usually by a veteran):
They shall not grow old as we who are left grow old.
Age shall not weary them, nor the years condemn.
At the going down of the sun and in the morning,
WE WILL REMEMBER THEM