What is Data Mart?
A data mart is a smaller, focused slice of a data warehouse built for one team or subject, such as sales or finance.
Data Mart explained
A warehouse covers the whole organisation, which can be overwhelming for someone who only needs one area. A mart exposes just the tables and measures that team uses, named in their vocabulary.
Marts keep reports consistent within a team, make access control simpler (finance sees finance data) and usually perform better because they hold less data.
Example
A sales mart contains deals, accounts, reps and quotas, with a ready-made definition of 'closed won', so every sales report uses the same logic.