Why, and on what terms, was the alliance system formed, and what were its implications?

By alliance systems, I am assuming you mean those that led to World War I. Austria-Hungary and Germany signed an alliance system because Austria-Hungary feared an attack by Russia. Russia signed an agreement with France of mutual support in the event of war because France was afraid of Germany. The Ottoman Empire signed a treaty with Germany because it hated Russia, and it wanted to regain land in the Balkans. Italy, initially part of the...

By alliance systems, I am assuming you mean those that led to World War I. Austria-Hungary and Germany signed an alliance system because Austria-Hungary feared an attack by Russia. Russia signed an agreement with France of mutual support in the event of war because France was afraid of Germany. The Ottoman Empire signed a treaty with Germany because it hated Russia, and it wanted to regain land in the Balkans. Italy, initially part of the Austro-Hungarian and German alliance, switched sides to the Entente when they were promised Austro-Hungarian land in the event of an Allied victory.

The terms of these alliances were not made explicitly clear at the time of their signing, and countries signed them out of fear of being attacked and not having enough support. The implications of these alliances were such that a small regional attack, such as a Serbian nationalist shooting an Austrian Archduke, then involved entire nations. For example, when Russia mobilized to support Serbia, Germany took it as a threat and mobilized. France, wanting to support its friend Russia, mobilized as well. Thanks to the alliance system, a war that should have been regional between Austria-Hungary and Serbia spread throughout the world and even included the United States.