Auschwitz I was after the war converted by the Polish government into a tourist attraction. The modern image of Auschwitz never actually existed as such -- the individual elements of the image are truth, but aspects of each of the three main camps, and from different periods, are blended together in a way that results in many inconsistencies with the historical reality.
The gas chambers shown to tourists (those at Auschwitz I) were not used to murder one million Jews; the gas chambers at Auschwitz II were used to do that. The Auschwitz I gas chambers were used to murder Soviet POWs and intellectuals, as part of an SS experiment to investigate more efficent means of murder than shooting. The gas chambers at Auschwitz I were converted to air raid shelters prior to 1942, i.e. prior to the commencement of the large scale murder of Jews. After the war the Polish government reconstructed them into gas chambers for the purposes of tourism.
Another famous aspect of Auschwitz, the "Arbeit Macht Frei" sign at the entrance, did in fact exist, but again is often presented in a way inconsistent with the historical reality. The "Arbeit Macht Frei" sign marked the main entrance to Auschwitz I prior to 1942. However, the mass murder of Jews did not commence until after the entrance marked with the sign was no longer used, a new rail entrance having been built instead; and Jews were most commonly taken directly to Auschwitz II, while the sign was at Auschwitz I.
Auschwitz I itself is to a large degree a reconstruction -- at the end of World War II it was left in a state of disrepair, and for several years after the war it was abandoned. In addition, some parts of the present-day town of Auschwitz are built on parts of what used to be Auschwitz I; thus parts of the current Auschwitz I boundary fence are a post-War reconstruction that is in a different location than the original fence, the original location of those parts having been built over.
Several authors have criticised the historical inaccuracies perpetrated upon Auschwitz by the Polish government, Jewish lobby groups, popular literature and Hollywood; see for instance: Tim Cole, Selling the Holocaust : From Auschwitz to Schindler; How History is Bought, Packaged and Sold (ISBN 0415928133 (amazon.com, search)).