Collection Classes

In AX collection classes have been designed for storing objects.
The classes are implemented in C++ to achieve the maximum performance (they are system classes).
collection classes are extended with Object class.

Array: Similar to the X++ language array type except that it can hold values of any single type, including objects and records. Objects are accessed in a specific order.

http://msdn.microsoft.com/EN-US/library/array.aspx

List: Contains any number of elements that are accessed sequentially.
Lists are structures that can contain values of any X++ type.
All the values in the list must be of the same type.

http://msdn.microsoft.com/EN-US/library/list.aspx

Map: The Map class allows you to associate one value (the key) with another value.
Both the key and value can be any valid X++ type, including objects.
The types of the key and the value are specified in the declaration of the map.
The way in which maps are implemented means that access to the values is very fast.

http://msdn.microsoft.com/EN-US/library/map.aspx

Struct:  A struct holds several values of any X++ type, to group the information about a specific entity.

http://msdn.microsoft.com/EN-US/library/struct.aspx

Set:The Set class is used for the storage and retrieval of data from a collection in which the values of the elements contained are unique and serve as the key values according to which the data is automatically ordered.

http://msdn.microsoft.com/EN-US/library/set.aspx

Changes to the Collection Classes in Dynamics AX 2012

http://blogs.msdn.com/b/x/archive/2011/09/07/changes-to-the-collection-classes-in-dynamics-ax-2012.aspx