Repositories are used to store and access data from different types of database. In its most basic form, a repository can be declared as a Java interface, and Spring Data will generate its implementation on the fly using opinionated conventions. These conventions can be overridden and/or complemented by additional configuration and, if required, some Java code. Spring Data also comes with some base Java interfaces, for example, CrudRepository, to make the definition of a repository even simpler. The base interface, CrudRepository, provides us with standard methods for create, read, update, and delete operations.