Short answers, pulled from the story.
The word Sudetenland did not exist as a unified territory before 1938. It emerged from the German language compound of Sudeten referring to the mountain range along the northern Czech border and Land meaning country.
King Ottokar II ruled from 1253 to 1278 and invited these settlers to populate less dense areas. Wenceslaus II followed him from 1278 to 1305 and continued the policy of inviting German immigrants.
The February 1921 census recorded 3,123,000 native German speakers living within Czechoslovakia. This group made up 23.4% of the total population yet faced growing cultural conflicts with Czech neighbors.
British Prime Minister Neville Chamberlain met Adolf Hitler in Berchtesgaden on the 15th of September 1938. They agreed to the cession of the Sudetenland without inviting any Czechoslovak representative to the discussions.
Between October 1 and the 10th of October 1938 the Sudetenland was assigned to Germany. The Czech government submitted to compulsion and promised to abide by the agreement on September 30.
By 1950 only 159,938 Germans remained out of the 3,149,820 who lived there in 1930. Around 500,000 people were expelled during the early phase between spring and summer 1945.