From time to time, there will be minor updates to Drupal core. If the release is designated as a security update, you should apply the update as soon as you can. Otherwise, you may choose to apply the update at any time to receive the bug fixes it contains. There are also major release upgrades, these you may want to apply so you have all the new and powerful features.
In Drupal 7 you have entities and fields, fields are attached to entities. A node is an implementation of an entity (the node module implements hook_entity_info() and other such hooks) so it can have fields.
Drush commands are provided for managing the migration process. Individual migrations can be imported and then rolled back. Running processes can be cleanly interrupted, individual source records can be migrated on their own, and performance can be instrumented.
drush help --filter=migrate provides a complete list of relevant commands