ContentSize is a property in the scrollview that represents the displayable area in the scrollview. If you have a scrollview, its frame is (0,0,320,480) and its contentSize is (320,960). That is, the entire content of the ScrollView is (320,960) in size, and you can see the content after (320,480) by sliding the ScrollView up and down.
ContentOffset is the offset of the current scrollView area vertices relative to the frame vertices. For example, in the last example when you dropped to the bottom, contentOffset would be (0,480), or y offset 480
ContentInset is the relation between contentView.frame.origin and ScrollView.frame. origin. For example, if the contentView frame is (0,30,320,480), then the contentInset is (0,30).