The concept of an iterator is fundamental to understanding the C++ Standard Template Library (STL) because iterators provide a means for accessing data stored in container classes such a vector, map, list, etc.
You can think of an iterator as pointing to an item that is part of a larger container of items. For instance, all containers support a function called begin, which will return an iterator pointing to the beginning of the container (the first element) and function, end, that returns an iterator corresponding to having reached the end of the container. In fact, you can access the element by "dereferencing" the iterator with a *, just as you would dereference a pointer.
To request an iterator appropriate for a particular STL templated class, you use the syntax
Take a look at this tutorial as well: http://www.cprogramming.com/tutorial/stl/iterators.html