The C++ STL(Standard Template Library) is a carefully constructed library developed by Alexander Stepanov, Meng Lee and others at Hwelett Packard.
The core of STL include container which represent data structure and algorithm which apply on the data structure and iterator which is the bridge between container and algorightm.
This example comes from the book of "Addison-Wesley, Designing Components with the C++ STL, Electronic 3rd Edition.pdf". Its a simple list implements an iterator in the class.
Look at each function carefully and get a clear idea of the mechnnism you will understand the core of the STL.