Sandbox is a playground that an organization can use for development or testing purposes. In simple words, it is a copy of your production organization. It is completely isolated from your production organization; it means the action that you perform on your Sandbox is not affected in the production organization. Let's start with an example; Salesforce will not allow us to write or change the Apex code in the production organization. In that case, the organization can use the developer Sandbox to develop the application by using the Apex code and move it from one production to other when it's ready to be deployed.
Salesforce comes up with a few types of Sandboxes for your organization. You can use them as per your requirements. When you purchase licenses from Salesforce.com for your organization, you can negotiate and get multiple Sandboxes at that time. Salesforce also allows you to buy a Sandbox any time you need it. The various types of Sandboxes are as follows...