Intent: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation [1]. 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.[1] 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.[1] for more information on the Iterator (Behavioural) Design Pattern.

Written by Bobby Nedelkovski
MathWorks Australia
Copyright 2009-2010, The MathWorks, Inc.

Reference:
[1] Gamma, E., Helm, R., Johnson, R. and Vlissides, J. Design Patterns : Elements of Reusable Object-Oriented Software. Boston: Addison-Wesley, 1995.