Intent: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation . This design pattern is also known as Cursor.
Motivation: An example of an aggregate object is an instance of the List ADT. Consequently, an iterator can be used to traverse the elements of a list with a set of high-level abstract operations. These operations may be implemented within the List ADT but as Gamma et al. wonderfully puts it, the key idea in this pattern is to take the responsibility for access and traversal out of the list object and put it into an iterator object - given the iterator describes behaviour whilst the list describes a collection.
Implementation: The Iterator abstract class is used purely to specify the requirements of its concrete implementation. This abstraction could be implemented as an external/active or internal/passive iterator -
External := the onus is on the client to advance the traversal and request next elements.
Internal := the client can supply an operation to the iterator to perform over every element of a collection
Refer to pp.257-271 Gamma et al. for more information on the Iterator (Behavioural) Design Pattern.
Written by Bobby Nedelkovski
Copyright 2009-2010, The MathWorks, Inc.
 Gamma, E., Helm, R., Johnson, R. and Vlissides, J. Design Patterns : Elements of Reusable Object-Oriented Software. Boston: Addison-Wesley, 1995.