A star join is a commonly used data warehouse query that runs against a star schema database, which consists of a large table (also known as a fact table) surrounded by dimension tables.
Fact tables typically include two types of columns: one that includes measurements, metrics, or facts about the business process, and another column that includes foreign keys to dimension tables. Dimension tables usually include descriptive attributes. For example, a fact table may include information about the number of hiking books sold by a particular author on a particular day. The corresponding dimension table includes the address, age, and gender of the author.
See Performance and Tuning Series: Query Processing and Abstract Plans > Joins: Retrieve Data from Several Tables > Query Plan Optimization with Star Joins.