Sandbox is a copy of the production organization. You can create multiple copies of your organization in separate environments for different purposes such as development, testing and training, without compromising the data and applications in your production organization.

Following are the different types of Sandboxes in Salesforce.com:
Developer Sandbox: A Developer sandbox is intended for development and testing in an isolated environment. A Developer Sandbox includes a copy of your production org’s configuration (metadata).

Developer Pro Sandbox: A Developer Pro sandbox is intended for development and testing in an isolated environment and can host larger data sets than a Developer sandbox. A Developer Pro sandbox includes a copy of your production org’s configuration (metadata). Use a Developer Pro sandbox to handle more development and quality assurance tasks and for integration testing or user training.

Partial Copy Sandbox: A Partial Copy sandbox is intended to be used as a testing environment. This environment includes a copy of your production org’s configuration (metadata) and a sample of your production org’s data as defined by a sandbox template. Use a Partial Copy sandbox for quality assurance tasks such as user acceptance testing, integration testing, and training.

Full Sandbox: A Full sandbox is intended to be used as a testing environment. Only Full sandboxes support performance testing, load testing, and staging. Full sandboxes are a replica of your production org, including all data, such as object records and attachments, and metadata. The length of the refresh interval makes it difficult to use Full sandboxes for development.

Sandboxes Available Per Edition:

Note: You can buy more sandboxes for any edition except Developer Sandbox, which is bundled with add-on sandboxes of other types.

Sandbox Feature Quick Reference: