This is the Memorial to the Murdered Jews of Europe - it's commonly referred to as the Holocaust Memorial, but there are separate memorials (or plans to build them) for other groups of victims.
It's a full block of giant cement rectangles, on undulating ground and at various heights. At the edges they're a few inches high; as you walk to the middle, they're several feet over your head. It's on prime real estate a block from the Brandenburg Gate.
I frankly didn't get much out of the above-ground part of the memorial - both times I was there, European teenagers were chasing each other around the blocks. The underground museum, though, is powerful. The shapes of the blocks above extend through the ceiling and tell some of the personal stories. Then there's one room where a recording tells the stories of people who died; there isn't enough known about most people to talk about, but they say if they did tell each victim's story, at this rate it would take nearly seven years to finish.
Going to Auschwitz was awful, but this was the place that made the Holocaust most real. It's important to me to understand the geography of events, and through the room at the memorial that showed photos and maps and explanations of different concentration camps (and mass murders outside of concentration camps) - the ones you've heard of and many more you haven't - I put it all together in my head. It wasn't pleasant.