What did the German people know about what was happening in the camps?

How much Germans knew about the camps

The first concentration camps were intended mainly for opponents of the Nazi regime, and enough was known about conditions to deter opposition. Many Germans were afraid of ending up in a concentration camp. There was even the well known saying, "Dear God, make me mute, so that I won't get sent to Dachau". (Lieber Gott, mach mich stumm, dass ich nicht nach Dachau kumm').
The existence of the extermination camps was also known in outline, though many Germans didn't want to know about the holocaust and took the line of 'hear no evil, hear no evil'.  

Answer #2

On the other hand even opponents of the regime (like Anna Seghers) are reporting that the concentration camps were declared as "correction camps" for dangerous individuals... fact is, nobody can really say how much this or that German knew about all this... I can hardly believe it when they say they didn't know anything - in the same time I can imagine the soldiers of the Wehrmacht, hurrying from one place to another were surprised - as they reported themselves - when they heard the rumours later on... some of them didn't see deportations, etc. and burning bodies could be covered by saying "we burn our deads after the bomber attacks" later in war.
So it's hard to say - the first answer gives an important idea. It's of course true that some people that had delivered their own family members or neighbours to GESTAPO (secret state police) pretended later they didn't know anything about that. But it wouldn't be correct to call a German a liar if he/she would tell you, they didn't know what happened - it's just hard to say...  

Further points

Obviously, the Nazi regime tried hard to keep the genocide of the Jews secret, but some information did get out, for example, some soldiers home on leave from the Eastern Front, did talk a little about mass shootings and so on.
As for the ordinary concentration camps, they were feared.
On the more general issue of what was known, here is what the historian Helga Grebing (born in 1930) wrote in an early, respected short history of the Nazi period:

'Protective custody, the concentration camps, the boycott of Jewish businesses, etc., emigration, persecution, the Yellow Star, the treatment of prisoners of war and slave labourers from Eastern Europe - nobody who lived in the Third Reich can claim to have known nothing, absolutely nothing, about all that'. […]

'In 1945 the mass of silent, inconspicuous fellow-travellers claimed never to have had anything to do with Nazism'.

Helga Grebing, Der Nationalsozialismus: Ursprung und Wesen, Günther Olzog Verlag, Munich, 1964 pp. 130-31. (First published in 1959).