A sandbox is a copy of your production environment that can be used for various purposes such as development, testing, or training. The three types of sandboxes are developer, configuration-only (config-only), and full (full-copy). The login page for sandboxes is https://test.salesforce.com/.
Questions related to setting up, configuring, administering, using or anything else about sandboxes should have this tag.
The three types of sandboxes each have different limitations and their availability is dependent on the type of license the organization has purchased. The production data (i.e., records) are not copied to developer and configuration-only sandboxes; however, they are copied to full copy sandboxes. Developer and config-only sandboxes can only be refreshed from production once per day whereas a full-copy sandbox can only be refreshed every 29 days.
Changes made in sandboxes do not affect each other or production and changes made in production do not affect the sandboxes. Changes can be moved between sandboxes using Change Sets, the Force.com Migration Tool, or the Force.com IDE. Not all changes are supported by the metadata api so they must be manually reproduced. See Which Metadata Components are Accessible and Components Available in Change Sets for more information on which changes must be manually administered.
Some additional information can be found in the following pages: