There are three kind of NS accounts : Development, Sandbox, Production.
From the functionality side they are the same. Sandox is copy of production database. An admin has permission to initiate refreshing data (copy) from Production database (account).
For you as developers there are a few important notice : Sandbox account never send email to the real receipt address, to prevent "test" or fake documents to be sent to the real customer or person. It always will be redirected back to sender