Origins
There are many theories as to how Silisia 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, a Germanic tribe like the Vandals and the (Celtic) Germanic Lugier or Lygier who all lived south of the Baltic Sea in the Elbe?, Oder, and Vistula river area. The "Silesia is Poland" argument is based on etymology and the fact that most of the place-names in the area are definitely of Slavic origin.
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 Silinger, a Germanic people. 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.
When we speak of Silesians, we are talking about the Slavic, and usually Polish, people who inhabited the area from the Early Middle Ages onward. When we want to speak of the Germanic tribe or people known as the Silingii, we leave the name intact, so as not to confuse the two.
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.
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
Since 1945 Silesia has again belonged to Poland. The Communists ousted most of the German population of Silesia. Some Germans managed to stay in their homeland, when it was taken over by Poland, mainly in Opolskie voivoidship.