DITA topics are the basic units of DITA content. Each topic deals with a single subject.
Generic topics are unspecialized single-subject articles. The generic topic provides the base for other specialized topic types (e.g. task, concept), and also lets you author content that does not belong in existing specialized types.
Concepts answer "What is..." questions. They provide background information about a product, interface, or task. The concept is a specialization of the generic topic.
Tasks answer "How do I?" questions. They describe what steps to take in order to achieve a particular aim. The task is a specialization of the generic topic.
References provide quick access to facts. They describe regular features of a subject or product, such as commands in a programming language, ingredients for food recipes, bibliographic lists, etc. The reference is a specialization of the generic topic.
Each glossary topic defines a single sense of one term. Besides identifying the term and providing a definition, the topic can identify related terms. The glossary is a specialization of the concept topic.
Composite (or ditabase) is a compound DITA document type. The <dita> element is a top-level container for multiple topics of different types ( generic topic, task, concept, reference and glossary), which can go in any sequence.
A collection of DITA topics can be organized by means of DITA maps.
You can find more information about DITA topics at: http://docs.oasis-open.org/dita/v1.1/OS/archspec/topicover.html.