What were the tensions in the Balkans?

Very basically...

The Balkans issue stems from the so-called "Bosnian Crisis" of 1908-09 and the "Balkan wars" of 1912-13

The wars were an important step towards WWI. Austria-Hungary was worried by the sudden rise of Serbia. A-H's ally, Germany was also concerned about what it saw as a Russian satellite state. Serbia's rise in power thus contributed to the two Central Powers' willingness to risk war.

Austria-Hungary had a massive empire and due to various treaties and military victories they administered the government of Serbia. Serbia didn't like this and there was a strong underground movement to gain independence.

The heir to the A-H empire (Franz Ferdinand) had wanted to visit the Serbian capital on what we might call a PR exercise. He wanted to show his face and demonstrate A-H's power over its smaller neighbour.

Long story short, due to a series of mistakes and alot of random luck on behalf of the assassins (That's the so called Black Hand Gang), the archduke was killed and A-H decided to use this opportunity to take full control of the country.

A-H told Serbia to surrender some freedoms and liberties or be occupied. Russia told A-H to back off or face war. Germany told Russia to back off or face a war with 2 nations. France told Germany to back off or face a war with 2 nations on 2 fronts. A-H called every ones bluff and invaded thus sparking the other declarations of war. The UK was eager to stay well out of it as it was far more interested in making money from the empire and didn't see the point of getting involved... until a small insignificant treaty from the 1830's popped up that stated that the UK would guarantee the neutrality of Belgium in any land war. The UK told Germany to back off Belgium but the Germans didn't take them seriously... Oh well it'll all be over by Christmas :D