"The nationalist not only does not disapprove of atrocities committed by his own side, but he has a remarkable capacity for not even hearing about them." "Actions are held to be good or bad, not on their own merits, but according to who does them, and there is almost no kind of outrage — torture, the use of hostages, forced labour, mass deportations, imprisonment without trial, forgery, assassination, the bombing of civilians — which does not change its moral colour when it is committed by ‘our’ side." George Orwell, "Notes on Nationalism" 1945 
Nationalism is an ideology that promotes popular attachment to the nation, and thus the state.
Often, this is constrained to being constructed from one or more of four characteristics which identify members of the nation: language, race, religion, and common historical experience. Patriotism is then differentiated from nationalism by stating that it promotes popular attachment to the state and its secular institutions rather than to the nation. The distinction between nation and state here is a nice one, however. The machinery of government is often disliked and distrusted by people who describe themselves as "patriots".
Patriotism is actually devotion to and desire to defend one's country, not a sense of inherent superiority. That is nationalism.
What is currently often labeled as patriotism is a form of nationalism which extends beyond defence to aggression and treatment of the inhabitants of other countries as inferior. The reason for the use of a different word is that nationalism is associated with empire, royalism, Nazi Germany and contemporary far-right groups. Re-branding it as Patriotism makes it more palatable.
Nationalism (and patriotism) have been abused by elites to politically mobilize the populace at all levels. Historically they have done this by
- exploiting past grievances
- exaggerating threats
- reinforcing false perceptions of threats
- driving the nation towards war
- emphasising the necessity of unity
- criticising nations as single entities
- criticising distinguishable sections of the population
- making claims upon national territories
- making claims about the (often divine) destiny of the nation
- the assertion and implication that foreigners are inferior
Nationalism and patriotism lend themselves to authoritarianism because they promote non-rational attachments over actual assesment of facts.
Types and examples of nationalism
- classical nationalism -- Romanian nationalism, Hungarian nationalism
- pan-nationalism -- Pan-Slavism, Pan-Germanism
- diaspora nationalism -- Zionism, Palestinian nationalism
- nationalism based on divine uniqueness -- Americanism, British Imperialism, Zionism
- Benedict Anderson. 1998. The Spectre of Comparison Nationalism, Southeast Asia, and the World. London: Verso. ISBN 1859848133.
- Maria Hsia Chang. 2001. Return of the Dragon: China's Wounded Nationalism. Boulder, Colorado: Westview Press. ISBN 0813338565.
- Mark Juergensmeyer. 1993. The New Cold War: Religious Nationalism Confronts the Secular State. Berkeley, California: University of California Press. ISBN 0520086511.