Both are same,but strictly speaking it is a Managed Bean.
Managed Bean is "Simply a Java Class in which User Interface specific methods can be written".
The reason for the two terms is that if a Managed Bean has a one to one mapping with a page & includes methods to access the UI components on that page,Then you can think of that Managed Bean as "backing" that page[i.e., Backing Bean].
Here Backing Bean Scope is limited to that page but Managed Bean scope is more than one.
Here One to one Mapping Means"Allowing the pairing of each page uniquely with a Java class"