Three types of Gateways:

• Gateway stored volumes : Keep your entire data onsite , storage gateway will back up this data asynchronously to Amazon S3. used to have inexpensive offsite back up option where you can restore the data locally or from EC2 instance. You can activate the gateway and assign it to your account.
• Gateway Cached Volumes : Most frequently accessed data stored locally. The entire data store set is stored in S3. No access if there is no internet.
• Gateway Virtual Tape Library (VTL): have collection of virtual tapes, each virtual tape is backed by Amazon S3 or Glacier, virtual tape exposes a iSCSI interface which provides your backup application with online access to the virtual tapes. supported by NetBackup, Backup Exec, Veam, etc.