Topics
A topic is a unit of information with a title and some form of content, short enough to be specific to a single subject or answer a single question, but long enough to make sense on its own and be authored as a unit.
Maps
DITA maps are documents that collect and organize references to DITA topics to indicate the relationships among the topics. They can also serve as outlines or tables of contents for DITA deliverables and as build manifests for DITA projects.
Specialization
Specialization allows you to define new kinds of information (new structural types or new domains of information), while reusing as much of existing design and code as possible, and minimizing or eliminating the costs of interchange, migration, and maintenance.
Generalization
Specialized content can be generalized to any ancestor type. The generalization process can preserve information about the former level of specialization to allow round-tripping between specialized and unspecialized forms of the same content.
Structural versus domain specialization
Structural specialization defines new types of structured information, such as new topic types or new map types. Domain specialization creates new markup that can be useful in multiple structural types, such as new kinds of keywords, tables, or lists, or new attributes such as conditional processing attributes.
Integration
Each domain specialization or structural specialization has its own design module. These modules can be combined to create many different document types. The process of creating a new document type from a specific combination of modules is called integration.
Customization
When you just need a difference in output, you can use DITA customization to override the default output without affecting portability or interchange, and without involving specialization.