There were something like 15,000 concentration camps across Nazi-occupied Europe in total, and they were used for lots of things and held lots of different sorts of people.
It doesn't take long with Google to flesh out the details, of which there are many, and there are worse ways to spend an hour this weekend than to remind ourselves.
The camps' extinction was bought at a very high price. Knowing about them and thinking soberly about how they happened is as respectful towards those who paid that price as wearing the poppy.
R