Define:
The iterator pattern is a design pattern in which an iterator is used to traverse a container and access the container's elements. 
The iterator pattern decouples algorithms from containers; in some cases, algorithms are necessarily container-specific and thus cannot be decoupled.

Usage:

You work with a variety of collection types
You want to be able to traverse various kinds of collections without knowing about their internal structure
You don't want to bloat every collection interface with traversal operations