A Sandbox in Salesforce is a snapshot of your production environment on the date you create or refresh it. A Sandbox instance is created in a separate environment for a variety of purposes such as development, testing, and training, without affecting the data and configurations in your Salesforce production instance.
Some people may consider Sandbox as a backup of production data and configuration at a point of time, but this is wrong because you usually will use Sandbox for other purposes such as coding, testing, quality assurance, and staging. Sandboxes should not be considered as data backups for the following reasons:
Full Sandbox refresh is limited to once every 29 days.
The full Sandbox is only included in the Unlimited and Performance editions. For all others, it has to be purchased as an add-on to your subscription.
The Sandbox should be nearly identical to your production data and configuration. You need to refresh the Sandbox to apply the latest production configuration...