The eNotes Guide to Nationalism says the following:

Nationalism is the doctrine that "the rulers should belong to the same ethnic (that is, national) group as the ruled" (Gellner, 1983, p. 1). The doctrine assumes that a ruler belonging to an alien nationality or ethnic group is not fully legitimate. However, the inverse formula is a sure recipe for ethnic cleansing, mass deportation, and genocide: to claim that the inhabitants of a specific constituency must share the same ethnic lineage as its leaders is effectively to give full legitimacy to the mass expulsion of different ethnicity and the drastic redrawing of boundaries to suit the group's pedigree. Nationalism also holds that "nation and political power should be congruent" (Gellner, 1983, p. 1). 

Nationalism can be very hard to define.  There are a number of ways to look at it and define it.  Let us look at two of them.

First, nationalism can be seen as the feeling that one is connected to the rest of the people in one’s country in an important way.  This sort of nationalism can be seen as a relatively positive thing.  It is said to have gotten its start in Europe around the time of the French Revolution.  At that time, people are said to have started to identify with their countries more than with the persons of their monarchs.  This form of nationalism promotes national pride and national cohesion.

Second, nationalism can be defined more negatively.  In this view, it is a sort of chauvinism in which we believe that our own “kind” are better than other types of people.  Our own kind can be defined in many ways.  We can be nationalistic on the basis of our race or our religion.  We can be nationalistic on the basis of our country, thinking that our fellow citizens are better than the people of other countries.  This type of nationalism can promote hatred of others and can result, in extreme cases, in things like the Holocaust.