Not to be confused with Germanic peoples.
The Germans, or the German people, is an ethnos and a nation (in German language: Volk), defined less by citizenship or being a subject to any particular country than by a sense of sharing a common German culture and having a German mother tongue. In the world today, approximately 100 millions of people have German as their mother tongue.
After the unification of a German nation state in 1871, the connection to the German Empire and the succeeding Weimar Republic, Third Reich, West Germany, East Germany and present-day Federal Republic of Germany has become more prominent in the understanding of who is a German and who is not. This process is however not finished, and it's for instance today rather a matter of controversy whether historical persons are to be seen as Germans or Czech or Austrian or Swiss or Bavarian...
But not all Germans were included in the German Empire. The Germans of Austria, whose emperor traditionally had been leading all Germans, remained outside just like many Germans in Imperial Russia.
Today, it might be sensitive whether to call an Austrian a German or not. The idea of a common German ethnicity shared by people in Austria and Germany has lost popularity, not least due to the Nazi "Anschluss" in 1938.
The Germans are a Germanic people, that is held to have expanded from Jutland and the southern shores of the Baltic Sea before the Migrations Period. Their Indo-European ancestors had before that migrated slowly from the Black Sea region, and arrived in southern Scandinavia. Assimilation with other peoples is postulated, both with the prior inhabitants of Scandinavia and with peoples encountered on their way from Asia. Then Celtic peoples were assimilated during the expansion southwards from the Baltic.
After the Migrations Period, Slavonics expanded eastwards at the same time as Germans expanded westwards. The result was German colonization as far East as in Romania and Slavonic colonization as far west as to present-day Lübeck, at the Baltic, Hamburg (connected to the North Sea) and along the rivers Elbe–Saale further South. After Christianization, the superior organization of the Catholic Church lent the upper hand for a German expansion on the expense of the Slavs, giving the medieval Drang nach Osten as result. At the same time, naval innovations led to a German domination of trade in the Baltics and Central–Eastern Europe through the Hanseatic League. Along the trade routes, Hanseatic trade stations became centers of Germanness where German urban law (Stadtrecht) was promoted by the presence of large, relatively wealthy German populations and their influence on the worldly powers.
Thus people whom we today often consider "Germans", with a common culture, language and worldview very different from that of the surrounding rural peoples, colonized as far from present-day Germany as Bergen (in Norway), Stockholm and Wiburg (in Sweden), and manywhere in Russia.
The "German" Holy Roman Empire was however not in any way exclusively German, and its course became much different than that of France or Great Britain. The Thirty Years War confirmed its dissolution; the Napoleonic Wars gave it its coup de grâce.
The reaction was a strong ethnic nationalism that emphasized, and sometimes overemphasized, the cultural bond between Germans. Alloyed with the supreme standing of German science at the end of the 19th century, and to some degree enhanced by Bismarck's military successes and the following 40 years of almost perpetual economic boom (the Gründerzeit), it gave the Germans an impression of cultural supremacy, particularly compared to the Slavs.
The Divided Germany
The idea that Germany is a divided nation is not new and not peculiar. Compared to the neighbors France, Russia, Sweden and Denmark it was obvious and true. Since the Peace of Westphalia, Germany has been "one nation split in many countries". The Austrian–Prussian split, confirmed when Austria remained outside of the 1871 created Imperial Germany, was only the most prominent example. Most recently, the division between East Germany and West Germany kept the idea at life.
The thought of a weak split nation gave birth to the idea of the advantage by unification. With Prince Bismarck as the great example, the Nazis went all the way and wanted to unite "all Germans" in one realm, which met a certain resistance among the Flemish and the Austrians, and much more so among the Swiss and the Dutch, who mostly were perfectly content with their perception of separate nations established in 1648.
Protestant Reformation started in the German culture, and Germans are both Protestants and Catholics. The late 19th century saw a strong movement among the Jewry in Germany and Austria to assimilate and define themselves as à priori Germans, i.e. as Germans of Jewish faith. In Conservative circles, this was not always quite appreciated, and for the Nazis it was an anathema. After the Nazi rule led to the annihilation of all domestic Jews, the controversy today is over the Gastarbeiter and later arrived refugees from ex-Yugoslavia, who often are Muslims.
Jus sanguinis still has a strong position in German law, but in the public debate it's increasingly argued support for jus soli along the line that immigrant-children are no immigrants themselves, why they should be considered Germans of equal rights and value as other Germans. Hence a growing number of Germans are of Muslim faith.
Historical persons like Mozart, Kafka and Copernicus might be called Germans, or might not. In the German mind, they belong to the German culture, which is what decides if someone is considered a German or not, at least in a historical context. But Germans also know that Austrians and Poles see things differently.
The Dutch and the Flemish has another (standard) language, so they constitute no real problem.