A collection of events, decisions, policies, and so on which might have changed the course of WWII - not necessarily Axis wins, but maybe prolonged wars, shortened wars, less deadly wars, and so on.
World War II is a war in which much of the world's peoples engaged in a bloody struggle for power, land (“Lebensraum”), and ideology (fascism, capitalism, communism). It was also rooted in nationalism based on racism and social darwinism and in dislike of atheism, communism, and differing religions (especially Judaism).
It is generally said to have started in 1939, when Germany attacked Poland, and Britain and France declared war on Germany in turn. But the war of Japan against China, which also plays an important role in wwii, already started in 1937, so did the more aggressive part of the German policy of taking back territory lost in World War I and unification with former territories of Austria-Hungary.
After Germany advanced into France very successfully, Italy joined the war on the German side. The Soviet Union used the opportunity to take land from Finland and to conquer the Baltics and eastern parts of Poland.
In a foreseeable match of industrial might and technological advantage, the Allies quickly reduced the Axis powers to piles of rubble, and the war ended in 1945 in Europe, while Japan gave up two months later.
World War 2 is often used in alternate history because of its potential to transform the world as we know. It's also popular because of the appeal of the concept “good vs evil”. However, morality is not a word to be used about a war where Stalin was technically a good guy. A thread full of discussion on the German Third Reich's potential to win can be found here.
Abbreviations: WWII, wwii, WW2, ww2.