User exits in Informatica MDM allow custom Java code to extend functionality at specific points in batch or API processes. A user exit is called by MDM Hub to run user-developed code with input parameters. Sample code is provided for a post-landing user exit that prints values, connects to the database, and calls a custom procedure. The different types of user exits are described for various MDM processes like staging, loading, matching, and merging. Common uses of user exits include data transformation, reject processing, loading related data, and manipulating child records.