Nationalism is a word used with different meanings, sometimes it means no more than that some people belonging to one nation want to be independent of another nation as in Welsh Nationalism and Scotish Nationalism. Most frequently nationalism means extreme nationalism or moderate nationalism.
Extreme Nationalism or jingoism is a Right Wing belief that your nationality, religion and or race etc. are awesome and that others don't deserve to exist. This is an extreme and a warmongering version of Patriotism.
This includes: hating other languages, hating other countries, Holocaust denial, War with other countries, claiming that anyone who disagrees with you is not a Patriot, hating people who don't follow your religion (Christianity in the West, the wrong type of Christianity in Ireland or Former Yugoslavia, Judaism in Israel, Islam in the Arab World), Shia Islam in an area where the majority are Sunnis, Sunni Islam where the majority are Shiites, voting Republican and plenty of other nonsense.