Nationalism
Nationalism refers to often right-wing ideologies and movements that promote the interests of a particular nation. There are numerous forms of Nationalism. Nationalism is sometimes confused with Patriotism, as often, especially in the United States of America, patriotism can easily blur into Nationalism.
Leftists often stand in opposition to nationalism. There are a number of reasons for this. Nationalism is always founded on a shaky narrative of history, where contradictions within a cultural group are erased in favour of a mythologized common identity. Next to this, nationalism will be used to distract oppressed populations into attacking external enemies rather than their immediate oppressors.
Despite this there are nationalist movements that get supported by many leftists, depending on the interests involved in them. Some nationalisms, for instance, try to break away from imperialist powers and bring sovereignty to previously oppressed people. (such as Irish nationalism, Scottish nationalism and many post-colonial struggles). Leftist support for these movements is generally based on the principle of national self determination.
It is a mistake to see leftist internationalism as simple anti-nationalism. Rather than negate authentic national identity, internationalists want to see a solidarity among different nations, empowering all of them.
Forms of Nationalism
Fascism
Fascism is a Hyper-Nationalist rightist ideology. It is very focused on the State.
Nazism
Fascist ultranationalism is based on the idea of a mythologized "nation," generally tied to some form of racial identity. One example of this is the "Aryan" of Nazism.