Amazon S3 is a web storage service by Amazon. Essentially, you get buckets you can store objects (files) in, and others can access those objects directly through the S3 service or via the web. This is particularly useful for hosting images and video...the storage and bandwidth go to their server farms, easing the load on your own server. Space and bandwidth are rather cheap ($0.15/GB for storage and $0.10/GB for bandwidth as of this writing), and you pay only for what you use.