For visitors coming from the US, Canada, Australia and similar (migration-based) countries, European identities might seem very strange: While in many non-European countries the term “nationality" is the very same thing as “citizenship", this is not how things are seen and done in Europe.
If we take Canada, for example, then being Canadian is the same as having Canadian citizenship - not having Canadian citizenship necessarily means not being Canadian. In Europe, however, things are much more complicated: Nationality and citizenship are two different things, that often overlap, but sometimes do not - namely in the case of national minorities. It is possible, for example, to be a citizen of Poland without being a Pole - and it is also possible to be a Pole without being a citizen of Poland. To put it simply, European identities are much more complex.
Why is it so different from the US, Canada or Australia? This major difference in the perception of identity can be traced back to a simple difference in the understanding of the state itself:
In Canada - if we stick to this example - the notion of a Canadian nation results from the Canadian statehood. Without the latter, there would be no Canadian nationality, because the idea of the nation is based on the existence of the state. In other words: First there's the state, and only because of the state, there's also a nation.
In Europe, it's exactly the other way around. First there's a nation. The nation might or might not have a state of its own, but its existence as a nation does not result from statehood and doesn't depend on it (for example: the Polish people in the 19th century, as there was no Polish statehood). If the nation does have a state, then the state is there because of that specific nation. The state is based on the existence of the nation and it basically exists for that particular nation. Hence the very European idea of nation-states, such as Poland (for the Poles), Hungary (for the Hungarians), Romania (for the Romanians) or Germany (for the Germans), to name a few.
Having said that, no nation-state in Europe is 100% identical with the nation for which it exists. For example: Not all the people who live in Hungary are Hungarians, and not all the Hungarians live in Hungary. Actually, there are substantial Hungarian minorities in neighboring countries such as Slovakia and Romania, which in turn means that not all the citizens of Slovakia or Romania are actually Slovaks or Romanians.
In other words: Nationality and citizenship are not the same in Europe. They are two different characteristics. And while they often overlap, they don't “have to". This means that European nation-states have, by definition, national minorities.
“National minorities" are, well, minorities - i.e. groups that differ from the majority. Being a national minority means that the difference lies in the national identity: While in Canada, all citizens of Canada are “Canadians", not all citizens of Poland are Poles. So basically, there are people who live in Poland (often for many generations), are citizens of Poland and yet differ from the Polish majority by having a different national identity.
This complexity has lead to many conflicts in modern European history. As a result, Europe now has an international convention for the protection of national minorities. The convention is implemented differently by each state. In Germany there are four officially acknowledged national minorities: The Sorbs, the Frisians, the Danes as well as the Roma and the Sinti (the latter are often referred to as Gypsies). Similarly, other European states, such as Poland, the Ukraine, Romania, Hungary, Slovakia and the Czech republic, officially acknowledge the Germans as a national minority. In fact, Germany has - like Israel - its own “law of return” for German minorities, and over the past decades, more than 4.5 million Germans “returned” this way to the historic homeland their ancestors had left centuries ago.
Obviously, there are many more national minorities in Germany than the four mentioned above, but only these four are officially acknowledged by the Federal Republic of Germany. Very significant minorities like the Turks have not been officially acknowledged. This is, politically, a hot potato. Germany is very interested in the “integration” of the Turkish minority into the German majority, which is a different way to encourage cultural and lingual assimilation. Yet the convention for the protection of national minorities obligates the majority to refrain from “policies or practices aimed at assimilation”. The state is actually required to support the minority in keeping its own national identity. An official acknowledgement of the Turkish minority as such would mean, for example, that it would be entitled to its own (bilingual) education system, which the German majority strongly rejects, as it would prevent their “integration”.
The Jews are acknowledged as a national minority in some European countries like Sweden, Poland or Romania. Interestingly, though unsurprisingly, Jews are not acknowledged as a national minority in Germany. This results from the heritage of Reform Judaism, which began here in the 19th century in order to help Jews to assimilate and to be perceived as a religious minority. Thus, Jews as a religious community are indeed an officially acknowledged, state-funded religion in Germany, but the Jews as a people are not.