What exactly is fascism? How did Hitler promote and spread it?

Fascism (created in the 1920s by Benito Mussolini) involves extreme nationalism, totalitarianism, irrationalism and warmongering. It also has a good deal of hero/leader-worship. Additionally, it is opposed to both socialism and capitalism. It is a tad fuzzy, resulting in modern political parties using it as an all-purpose insult, but in reality it is much narrower than such mudslingers want people to believe. Hitler expanded on fascism to create Naziism, an extreme form of fascism that was (among other things) more overtly racist. He attempted to fulfill and promote the ideology with mive war and genocide. This went over about as well as one might expect with the international community, which is why fascism, especially of the Nazi variety, is today a discredited ideology.