Silesia is the Latin and English name for a province which is situated nowadays in western part of Poland, and is divided into Dolnoslaskie (with capital in Wroclaw), Opolskie (capital: Opole) and Slaskie (capital: Katowice) voivoidship. The Polish name of Silesia is "Slask". Its German name is "Schlesien".
There are many theories as to how Silesia derived its name. These theories tend to fall along the lines of national interest. The "Silesia is part of Germany" argument claims that the name is derived from the Silingii, most likely a Vandalic people, who lived south of the Baltic Sea in the Elbe, Oder, and Vistula river area of Germany. The "Silesia is Poland" argument is based on etymology and the fact that the place-names in the area now are Slavic.
A third theory claims that the area was indeed "originally" (as far as they are the first people purported to have lived in the area) inhabited by the Silingii. When the Silingii moved from the area during the Migration Period, or Völkerwanderung, they left remnants of their society behind. The most evident remnants were in the place-names, which were adopted (in Slavic form) by the new inhabitants, who were in fact Slavic. These people became associated with the place, and were known as Silesians (using a Latinized form of the name), even though they had nothing to do with the Silingii.
Parenthetically, it should be noted that there is considerable debate among archaeologists and historians as to whether there is such a thing as a Celtic-Germanic people. Exhibits such as the one in Rosenheim (Bayern) certainly demonstrate that the Celts had an influence on the area; however, the movement of the Celts westward through Europe was such that there is little if any overlap between them and the Germanic tribes.
Moreover, the question of Germanic tribes and their relationship to place names is entirely chicken/egg. Traditional German historiography, most notably the works of Ranke, tend to argue a thing's inherent "Germanness" on the grounds that clearly work in a 19th century nationalist context, but hardly work for today's historians. To argue that the people living in Silesia before various Slavic peoples moved into the area were the same ethnic group as those living in Silesia in 1945 is insupportable. Of course they are not the same people. Not too many people live to be a 1000, except the Israelites, of course.
We need a better transition here -- perhaps some of the good info provided by H. Jonat and now residing on the Talk page could be re-worked?
History -- Middle Ages
History - Early Modern Period
Silesia in the Modern World
In 1945 all of Silesia was taken by Soviet Union troops. Stalin (the Four Powers?)assigned it to administration by Poland.
People of mixed origin (German-Polish families or ancestors) and some German Silesians stayed in their homeland. They were exposed to some forms of discrimination by the Polish communist authorities and by some Polish priests in the years after the World War II, who told the Germans they should learn Polish.
After the fall of the communist regime in Poland the Silesia Germans, now a minority in Poland,were guaranteed freedoms of democratic societies, they even have their representation in the Polish Parliament. During the years 1946-1989 thousands of people claiming German ancestors applied for transfer to the Federal Republic of Germany, which then became their new home. In part it was reunion of expelled families, Heimatvertriebene but in many cases it was also for economic reasons. West Germany welcomed warmly the German people from east of the Oder-Neisse rivers, because of the negative demographic growth. Some German Silesians stayed in their homeland, mainly in Opolskie voivoidship. They are now allowed to change their names back to the original German birth names, which were by force "Polanized" after 1945, when all personal identification such as birth certificates were taken from all expelled and from the Autochthones, the German citizens remaining in their homeland, now under Poland.