AWS S3 and AWS storage gateway

i am bit confused between AWS S3 and AWS storage gateway as both functions the same of storing data.Can anyone explain me with example of what is the exact difference between two services offered by Amazon

• AWS S3 is the data repository
• AWS Storage Gateway connects on premise storage to the S3 repository.

You would use Storage Gateway for a number of reasons

1. You want to stop purchasing storage devices, and use S3 to back your enterprise storage. In this case, your company would save to a location defined on the storage gateway device, which would then handle local caching, and offload the less frequently accessed data to S3.
2. You want to use it as a back up system - whereby Storage Gateway would snap shot the data into S3
3. To take advantage of the newly released virtual tape library - which would alloy you to transition from tape storage to S3/Glacier storage, without losing your existing tape software and cataloging investment.