In C++, the Standard Template Library (STL) provides a set of iterators that allow for efficient traversal and manipulation of container elements. An iterator is an object that points to an element within a container, and provides a way to access and modify the value of that element.

STL iterators can be classified into five categories:

1. Input iterators: These iterators allow traversal of a container in a forward direction, and provide a read-only access to the elements.

2. Output iterators: These iterators allow traversal of a container in a forward direction, and provide a write-only access to the elements.

3. Forward iterators: These iterators allow traversal of a container in a forward direction, and provide both read and write access to the elements.

4. Bidirectional iterators: These iterators allow traversal of a container in both forward and backward directions, and provide both read and write access to the elements.

5. Random access iterators: These iterators allow traversal of a container in any direction, and provide random access to the elements based on their position. They provide a rich set of arithmetic and relational operations, such as increment, decrement, addition, subtraction, comparison, and indexing.

STL containers provide a set of member functions that return iterators pointing to various elements within the container. These iterators can be used in a variety of algorithms and operations, such as sorting, searching, copying, and modifying elements. For example, the sort() algorithm can be used to sort a range of elements within a container, using random access iterators for efficiency.

STL iterators provide a standard and consistent interface that makes it easy to use different containers interchangeably in the same code. They also provide a powerful and flexible way to manipulate container elements, and can be used to implement custom algorithms and data structures.