Seems like a natural topic, but every search I tried turned up nothing.

Is there good, concise information aimed at the traveller, not pilot or air professional, on airplane types and designators? For example, I've flown on various B737, MD80, B747, B757, B767, B777, Saab 340, etc. but I don't know the difference between a -300 and a -400. Are the designators arbitrary?

How about tips for telling one plane from another when you're bored and staring out the window while taxiing?

Or maybe an explanation for the third digit in the Bxxx series? E.g., the 757 is smaller than the 747. And why are the first and third digits always the same? Similar questions for Airbus planes.