No one is the villain of his own story, so there are conflicting accounts of what World War I was like based on what country is telling the story. As Napoleon said, "History is a set of lies agreed upon," so there are several constant truths about the war, such as its causes, the technologies used, and the aftermath.
World War I had centuries of long-standing alliances between European nations that obligated each of them to assist their allies. The spark that is considered to have officially started the war is the assassination of Austrian archduke Franz Ferdinand. Germany aided Austria, Russia aided Serbia, and other countries took their respective sides.
World War I made use of the first 'modern' technology like automatic fire weapons, tanks, planes and artillery. Coupled with trench warfare, it made the war one of the bloodiest in the 20th century. Men would huddle in trenches, waiting to break the top and charge to the enemy trench amidst barbed wire, mustard gas and artillery bombardments. It could take months to advance a few miles.
World War I is considered to have set the stage for World War II, partially due to the Treaty of Versailles. This treaty placed almost full blame for the war on Germany and demanded reparations be paid to all nations involved.