Spring Data JPA implements the core functionality defined in Spring Data Common interfaces.

JpaRepository is the JPA-specific repository interface:

   public interface JpaRepository<T, ID extends Serializable>
   extends PagingAndSortingRepository<T, ID>, 
   QueryByExampleExecutor<T>     {

SimpleJpaRepository is the default implementation of the CrudRepository interface for JPA:

   public class SimpleJpaRepository<T, ID extends Serializable>
   implements JpaRepository<T, ID>, JpaSpecificationExecutor<T>