Do normal titans have humans in them as well? There are several humans which can transfer into titans at will. What about the normal titans? Hange said that titans do not weight as much as they should. Their only vulnerable area is where the real human body would be for shape-shifting Titans. They also look like humans, are there humans inside normal titans as well?
This was explained titans were humans at one point. The ones from the Ymir's village were turned into them against their will.  http://shingekinokyojin.wikia.com/wiki/Titans 

It was also hinted at by experiments with Eren that the more he uses shifting without resting the more he merges with the titan.  

Ymir also wandered aimlessy as a titan without ability to shift or control her actions for a while, indicating most titans are in the same situation as her.