migrate_d2d provides a framework based on the Migrate API for migrating content from Drupal 5, Drupal 6, or Drupal 7 sites into the Drupal 7 site where it is installed. As of the 2.1 release, It requires Migrate 2.6 or later. Besides addressing contemporary needs to migrate to Drupal 7, it has helped serve as a proof-of-concept for incorporating the migration approach into core as an upgrade path (https://groups.drupal.org/imp).
The core framework provided here is used by providing your own module, which will register instances of the migrate_d2d classes (or derivations of them). See migrate_d2d_example for one approach, where instances are registered when the Drupal caches are cleared (note that registration updates previously-registered classes with any argument changes).
With migrate_d2d 2.1, we introduced a wizard-based UI for defining your Drupal-to-Drupal migrations. The wizard is appropriate for non-technical users to configure and run the Drupal-to-Drupal migration, and/or when you don't have to do any special manipulation of data along the way.
Enabling the migrate_d2d_ui module will add an "Import from Drupal" subtab to the Migrate dashboard - visit that tab, enter the credentials for your source Drupal database (versions 5, 6, or 7), and follow the steps to define how your legacy content maps to the destination Drupal 7 site.