This module provides drush support for upgrading from Drupal 6 or 7 to Drupal 8. The 8.x-1.0 release of the module, which only runs with Drupal 8.0.x, also provides a web UI for the upgrade process (the UI has been added to Drupal 8 core for 8.1.x, thus is no longer provided in the 8.x-2.x branch).
The plan is for the drush support to be added to drush itself at some point - at that point this module will become obsolete.
To run a complete upgrade of your legacy Drupal site to the current D8 site:
drush migrate-upgrade --legacy-db-url=mysql://user:email@example.com/d6db --legacy-root=http://myd6site.com
UI (8.x-1.0 only)
After installing, visit /upgrade in your browser to access the interface.
It is important to note that this module only provides the front-end(s) to the upgrade process - if you see a problem with the results of your migration, this is almost certainly an issue with the core framework, and should be reported to the core issue queue with a component of "migration system".