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A sandbox is a nearly identical copy of your production environment available to Enterprise or Unlimited Edition customers. The sandbox copy can include data, configurations, or both. It is possible to create multiple sandboxes in your production environments for a variety of purposes without compromising the data and applications in your production environment.
Here is a great link that explains alot about testing environments and the limits attached to each type of sandbox: https://developer.salesforce.com/page/An_Introduction_to_Environments
