Study Curve

a series of entangling secret alliances
German militarism
the decay of European monarchies
a desire fro revenge from France

The "official" cause was that the Archduke of Austria (Franz Ferdinand) was assassinated by a Bosnian Serb. There were a series of confusing treaties that required countries to support one another if they were attacked, which is how so many European countries got involved what would probably have otherwise been a minor conflict.

However, this was mainly just an excuse by the major powers to fight over colonies like Jerry mentioned as well as to avenge past ills (such as the Franco-Prussian War in 1870).

There were a number of empires all fighting for additional territory world wide including Russian, German, British, French and Austria-Hungarian. I would love to go into more detail, just let me know if you need it.