As with any conflict, there were many conditions that led to the 'great war'. Alliances and distrust between countries, old rivalries, and the desire for global expansion all contributed to a conflict that raged for years and engulfed numerous countries.
This interesting podcast from the ABC features historians debating the causes of WWI.
For the true history buff out there you must check out Dan Carlin's extensively researched and extremely detailed podcast. The first episode (which clocks in at three hours) examines the conditions that led to the assassination of Ferdinand.