The Definition, Advantages and Disadvantages
A data mart is a subset of a data warehouse.
It is really similar to a data warehouse but limited in scope and purpose and is usually aligned with one department, function, application or business unit.
It could also be defined as a single domain aggregation used for reporting, analysis and decision support. A single domain or subject could be Customer or Supplier.
A mart could be extracted from a data warehouse or could feed a data warehouse.
Example of Use
An example application would be where the marketing department what to some predictive analysis on which sales locations would be best at selling a new line of products.