Moon Raker: The difference is size, and consequently price.
The C-class is the 'compact luxury' category, or if you're familiar with the generic size categories, then it's the 'D' category. (Its direct competitors are the 3-series BMW and Audi A4.)
The E-class is bigger, defined as the 'mid-sized luxury' or 'executive', or 'E' category. (Comparable to BMW 5-series or Audi A6.)
The S-class is bigger still, 'full-size luxury', or 'F' category. (Comparable to BMW 7-series or Audi A8.)
they all have a slightly different interior. E and S class is larger then the C. the C is tuned slightly more toward sport. otherwise they are all the same
different class, different engine, different economy, different looking, different price