Your Drupal site will consist of three environments:
A development enviroment with a URL of site-dev.umn.edu or site.dev.umn.edu
A staging environment with a URL of site-stg.umn.edu or site.stg.umn.edu
A production environment with a URL of site.umn.edu
where site is the alias of your site.
This site is usually used for developing code such as sub-themes or custom modules. This is the environment where the developer will upload, de-bug, and troubleshoot code.
When the developer is done with coding a site, you will move it to the staging environment for testing. In this environment contributors, site managers, and other stakeholders will verify that the code works as expected. In order to do this, they usually want to test with live data so you will need to copy the database from production to staging.
When you first migrate to Drupal, the staging environment is also where your migrated content will be loaded and tested before moving to production.
This is the public facing site where people go to consume your content. This is also where the contributors add content to the site.