Fascism is a political ideology with a strong belief in nationalism and autoritarianism which tends to be located in the far-left (and sometimes far-right) spectrum. Fascism was built under the basis of Socialism and there's no big difference between National Socialism and Fascism.
The term "Fascism" tends to be used to describe (usually as a despicable term) people or parties who showed a strong belief in only authoritarianism or nationalism. Especially nationalists are the ones most described as fascists. Another use of the term is when someone seems to disagree with Socialism, democracy, anarchism and similar ideologies that tend to be less authoritarian and nationalist. Most of the nationalist ideologies are built upon Fascism.