The Alliance system was one of the causes of World War I. Prior to the war there were two alliances. One was the Triple Alliance. This included Austria-Hungary, Germany, and Italy. The other alliance was the Triple Entente. This included Soviet Union (Russia), Great Britain, and France. The danger of these alliances is that if a member from one alliance declared war on a member from the other alliance, the conflict would quickly escalate. That is what happened in World War I.
When the next king of Austria-Hungary, Franz Ferdinand, was assassinated by a group of Serbian nationals, Austria-Hungary made a series of demands on the government of Serbia. When the government couldn’t meet all of these demands, Austria-Hungary, with the support and approval of Germany, declared war on Serbia. Since Serbia and Russia were very close friends and allies, Russia declared war on Austria-Hungary. Germany then declared war on Russia. France and Germany then declared war on each other. Eventually, Britain declared war on Germany when Germany invaded the neutral country of Belgium. Thus, what was originally a conflict between Austria-Hungary and Serbia quickly escalated into World War I, involving many countries throughout the world.
Before World War I, many of the major European powers were involved in two strategic alliances. The first of these two was the "Triple Alliance" or "Triplice." Formed in May of 1882, it consisted of Germany, Austria-Hungary, and Italy. The second opposing alliance was the Triple Entente, originating in bilateral treaties between France and Britain (the "Entente cordiale") and Britain and Russia.
In World War I, these alliances became the cores of the two opposing sides, with the Triple Alliance becoming the Central Powers and the Triple Entente becoming the Allied Powers.
The reason that the alliance system contributed to the First World War was that it meant that local conflicts, even minor border disputes, rather than remaining local in nature would draw in all members of both alliances. Thus conflicts between Austria-Hungary over Serbia drew in not only Russia, which had territorial ambitions in the area, but also their western European allies.