In World War I, there were two major alliances. The Central Powers included Germany, Austria-Hungary, and Turkey. The Allies included France, Great Britain, Russia, Italy, and the United States.
The purpose of an alliance is to have countries support your country in case of an attack, dispute, or crisis. When Serbian nationals killed Franz Ferdinand, the next king of Austria-Hungary, Austria-Hungary made many demands on the Serbian government. When the Serbian government couldn’t meet all the demands, Austria-Hungary wanted to declare war on Serbia. Before they did this, Austria-Hungary asked Germany if Germany would support them if they declared war. Germany agreed to do this, so Austria-Hungary declared war on Serbia. Serbia was protected by its alliance with the Russia, so they were helped in the war.
Once one member of one alliance declared war on one member of the other alliance, it then brought many other countries into the war. Suddenly a two-country conflict became a multi-nation conflict. Throughout the war, alliance members helped each other fight in the conflict.
World War I was a different kind of war because it was an eruption of pent up anger from a lot of the countries in the world. The Serbians and the Austria-Hungary Empire are looked to for the initial beginning of this war. After Archduke Franz Ferdinand was assassinated by a Syrian rebel, Austria-Hungry declared war on Serbia. Now it is important to understand that the two sides to this war are more or less ethnicity based with a strong sense of nationalism mixed in. Even though ethnic groups sprawled across borders, they still had a large sense of loyalty to their nation and people. So once Russia heard about Serbia, they mobilized their army. Germany, who had been long time allies with Austria-Hungry mobilized against Russia. At this point the other European countries snowballed into World War I.