Balkans - the area of Southeast Europe that included Serbia and much of the Austro-Hungarian Empire in 1914.
Prussian War - The Franco-Prussian War (1870-1871) saw the French suffer a humiliating defeat. They had to give the eastern provinces of Alsace and Lorraine to the new German Empire.France wanted revenge for defeat in this war, and to get Alsace and Lorraine back from Germany.
A.J.P. Taylor (1906-1990) was a prominent British historian in the Twentieth century.He wrote several major works, including 'The Struggle for Mastery in Europe 1848-1918' which says WW1 was inevitable and had been coming for a long time.
Imperialism - the desire and practice of building up an empire by conquering/colonizing other countries
Nationalism - the idea that people in a certain area should be free to chose their own leaders, rather than be part of someone else's empire and be ruled by foreigners.
Alliances - agreements between countries to help each other in a war
Kaiser Wilhelm - Autocratic Emperor of Germany in 1914. He liked Germany having a strong army and navy, and wanted Germany to be the leading world power.
Assassination of Franz Ferdinand - he was heir to the throne of the Austro-Hungarian empire. His murder by a Serb nationalist in June 1914 made Austria want to attack Serbia.
Schlieffen Plan - This was the German war plan in 1914. Russia and France were allies, so Germany would have to fight a war on 2 fronts.However,the Russian army was very big.The Schlieffen Plan was designed to beat France quickly,by attacking France through neutral Belgium, before the Russian army was ready for war, so the German army could be moved east to fight the Russians. (It failed).
Arms Race - a situation in the years before WW1 where every country was building up stronger and stronger armies and navies with more modern weapons as fast as they could.
These were all things that were causes of WW1.