Note: This blog post is based on Drupal 8.1.x. It is an updated version of a previous tutorial based on Drupal 8.0.x. While the concepts are largely the same as 8.0.x, a refactoring of the core migrate modules took place in Drupal 8.1.x (migrations will become plugins in 8.1.x). This updated tutorial updates the previous example to work with Drupal 8.1.x, as well as demonstrates how to specify a migration group and run the migration with Drush. If you're familiar with the previous tutorial, you may want to skip to the "Rolling up our sleeves" section below.
Even if you're only casually acquainted with Drupal 8, you probably know that the core upgrade path to Drupal 8 has been completely rewritten from the ground-up, using many of the concepts of the Migrate and Drupal-to-Drupal migration modules. Using the Migrate upgrade module, it is possible to migrate much of a Drupal 6 (or Drupal 7) site to Drupal 8 with a minimum of fuss (DrupalEasy.com is a prime example of this). "Migrate upgrade" is similar to previous Drupal core upgrade paths - there are no options to pick-and-choose what is to be migrated - it's all-or-nothing. This blog post provides an example of how to migrate content from only a single, simple content type in a Drupal 6 site to a Drupal 8.1.x site, without writing any PHP code at all.