Amazon S3 is a online storage service provided by Amazon. It is an internet storage system where you can achieve store images, web hosting, backup solution at a reasonable cost. It caters from small to enterprise business and it is widely popular.
The retrieval and pushing data is done using Web Services. they claim the service as high availability, highly reliably and scalable. Since it is using web services, web developers can extend the service to any platform easily and make services.
An example being a Image hosting service, a Travel Portal which contains lots of images, galleries etc can make use of Amazon S3 thus reducing the bandwidth costs, file serving requests to webserver.
Amazon S3 is made up of buckets and objects. All Objects are organized into Buckets and can assign a unique key and later can be retrieved by REST, SOAP or BitTorrent. Buckets and keys can be accessed by URLs.
To keep costs lower, many startups and small companies have embraced this platform thus reducing their costs. 37Signals.com uses S3 to store their data.
It can be used as a good backup tool for servers and also local machine and as it is internet based, can be accessed from anywhere in the world. Here is a list of amazon backup tools
S3 is very well documented. Smugmug a photo hosting service have posted amazon s3 savings in their blog. Amazon S3 can be accessed by web browser from this s3interface.com.
A popular service akamai is used for image hosting and many other types of storage, but when compared to amazon,amazon s3 is much cheaper in storage, when you have high bandwidth websites, you need to workout bandwidth costs against other services on the internet.