Bulding warehouse tutorial – very good tutorial, with a lot of information from basics.
Dimension hierarchies introduce formal hierarchies into a business model, allowing Oracle BI Server to calculate useful measures and allowing users to drill down to more detail. In a business model, a dimension hierarchy represents a hierarchical organization of logical columns belonging to a single logical dimension table. Common dimension hierarchies used in a business model are time periods, products, customers, suppliers, and so forth.
Dimension hierarchies are created in the Business Model and Mapping layer and end users do not see them in end user tools such as Oracle BI Answers or Interactive Dashboards. In each dimension hierarchy, you organize dimension attributes into hierarchical levels. These levels represent the organizational rules and reporting needs required by your business. They provide the structure that Oracle BI Server uses to drill into and across dimensions to get more detailed views of the data. Dimension hierarchy levels are used to perform aggregate navigation, configure level-based measure calculations, and determine what attributes appear when Oracle BI users drill down in their data requests.
There are two main categories of logical tables: fact and dimension.
Logical fact tables contain the measures by which an organization gauges its business operations and performance.
Logical dimension tables contain the data used to qualify the facts.
Source: Creating a Repository Using the Oracle Business Intelligence Administration Tool