How do C and C plus plus differ in terms of data abstraction and classes and structs?

They differ insofar as C does not use object-oriented programming at all -- there are no classes (only structures), therefore there was nothing to abstract. C++ (which literally means 'the successor to C') is an extension of C that primarily adds object-orientated support to the language. Everything you can do in C you can also do in C++, but with the added benefits of OOP you can do a whole lot more, more easily, including the creation of abstract data types.