What do the naming conventions of German cars represent like "C Class" and "E Class"?

Mercedes has C Class, E Class, etc.
Audi has A4, A6, etc.
BMW has 3, 5 and 7 series.

What are the differences between the various models of cars?

Does it mean, the more money someone has, the more expensive model they will buy?

Other answer:

DR + Mrs Bears face:
HI simple logic here with all three makes mercedes start with A series. as the basic level then Have B,C, and E as excellence level. the Bow like wise series 1, 3, 5 and series 7.
the audi like wise A1 A2 A3 A4 A5 A6 A8, so the all are made by AG group which now includes Porsche as well along with Seat, Volkswagen, Skoda, Bugatti and Rolls Royce as well as bentley , and also Mayback.
JetDoc:
There IS NO "Naming convention". Each manufacturer is free to name their cars any way they want.
Bill:
That's the case no matter how the models are named. Some automakers use letters and numbers so they don't have to translate the names.
boy boy:
as others say model series ..however normally the higher the number the larger the car
Bobby Jim:
As the letters and numbers advance, the luxury, size, and power of their models increases.
Ezra Church:
They do that crap just to confuse foreigners. Those crazy Germans.