What is the significant difference between car models?

For example,

What's the difference between BMW 1-7 Series and M-Series? They're both sedans right (well, M-Series have coupes). How do they classify them? What's the difference?

Is it all in the car's body? If it is, then why do Honda CRV's old and new model become CRV? I mean, they have a significant difference when it comes with appearance because the old model has the reserve tire at the back while the new model has it elsewhere.

Or it differs from the specs? If it differs from specs, then how can we say that the new BMW M3 is a M3 when the old model has a lower specs?

I hope you can understand what I mean. If not, please do write and i will clarify things.

There's huge difference between 1 series and 7 series… M powers are sportier models

I don't exactly understand. Are you asking why the model name remains the same throughout the years while the specs change?

You're trying to make something out of nothing. The car manufacturer can call them whatever they want.

Do you really mean the difference between all the hundreds of car models? All of them are different colors.

Google it

Comlicated becasue the series itself represents different class, comfort, engine size… And price…