To learn more about the Holocaust in general, please refer to these websites:
http://www.yadvashem.org
www.jewishvirtuallibrary.org
http://www.ushmm.org
http://www.projetaladin.org/holocaust/en/homepage.html
To learn more about specific concentration camps or extermination sites, please refers to the websites below. Many of the concentration camps have their own individual websites dedicated to memorializing the events of the Holocaust.
http://www.thirdreichruins.com/buchenwald.htm
http://en.auschwitz.org
http://www.kz-gedenkstaette-dachau.de/index-e.html