EdT has a higher concentration of scent ingredients than aftershave. You could say AS is watered down EdT. Some AS's have extra ingredients to soothe the skin, like oils.
The difference between Trumper etc. and department store colognes is mainly, I believe, that the classic cologne houses use less synthetic ingredients (so more natural EO's), which makes them more expensive, but also, IMO, much better. Think Trumpers, Penhaligon's and Floris VS. armani or D&G.
To me, many 'modern' dept. store colognes smell like toilet cleaners, nasty 'blue' and 'cool' scents without a hint of warmth or class....
For colognes, I'm really partial to TOBS: great scents with a long history, and not overpriced.