Cendrillon came from the word Cendre, which means ashes. As another word for ashes is cinders, Cinder was taken for the Cendr part of the name, and they put Ella behind it. In the German version Cinderella is called Aschenbrödel, literally "scullion,".
So, what is Cinderella's real name?
Cinderella's real name is Ella (Mary Beth Ella Gertrude) via the Disney version of the tale. However, in other versions she had many other names. The Grimm brothers' (they wrote the original) version is Cinderella (Aschenputtel). Hope this helps.
Why do they call her Cinderella?
The stepsisters called her that because she was forced to sleep in the cinders by the fireplace, leaving her dirty. "Cinderella" meant something like "little cinder girl". It's not clear what Cinderella's real name is supposed to have been. But since "Ella" is a modern first name it's a reasonable choice.