Auschwitz II (Birkenau) is the camp that many people know simply as "Auschwitz". It was the site of imprisonment of hundreds of thousands, and of the killing of over one million people, mainly Jews but also large numbers of Poles, and Gypsies.
Birkenau was largely destroyed by the Nazis in an attempt to cover up their crimes, so little remains except the chimneys for most of the buildings that the prisoners were kept in. The scale of the place is astonishing; as you can see from the photo, the chimneys stretch on as far as the eye can see across the sprawling 440 acre complex.
Bear in mind that three-quarters of the prisoners who were sent to Birkenau were gassed within a few hours of arrival, so all of these buildings were needed to house only the remaining quarter, who were utlized as slave labor in nearby industrial plants.
There are more photos of Birkenau here; obviously, some of you may find them trigger-y.
I did not get to make as many photos at Birkenau as I would have liked, as it was already getting quite dark (though it was only about 4pm Central European Time, the days here are very short suring winter) and my digital cam was running on fumes. In any case, there's no way that any photograph could possibly capture what it is like to be there.
What's commonly known as "Auschwitz" is actually three separate concentration camps.
Auschwitz I was first established in 1940 and served as the administrative hub for the whole complex (and is the current site of the museum exhibits).
At any time, the camp held between 13,000 and 16,000 inmates; in 1942 the number reached 20,000. It was the site of the deaths of roughly 70,000 people, mostly Poles and Soviet prisoners of war.
There are more photos of Auschwitz I here; obviously, some of you may find them trigger-y.