Note: The Migrate module documentation is, as befits a complex subject, always in need of improvement. The best way to contribute improvements is not comments on the documentation pages (unfortunately, there is no notification process to tell the authors of comments), but to submit a Documentation issue to the Migrate module issue queue.
For Drupal 8, see in-progress Migrate API in Drupal 8.
The Migrate module (V2) provides a flexible framework for migrating content into Drupal from other sources. It is important to recognize that the Migrate module is a developer's tool — if you do not have object-oriented programming skills in PHP, this module is not for you. The primary purpose is to provide the infrastructure for managing complex, large-scale site migrations. However, add-on modules can be developed on top of Migrate to provide a UI suitable for use by non-technical users for simpler migration cases — as an example, see the WordPress Migrate module for importing Wordpress exports into Drupal 7.