What is Amazon Aws? Is it hosting service provided by Amazon? Is Twitpic hosted by AmazonAws? How much it cost (I couldnt find on their site)
AWS is Amazon Web Services. It's the umbrella term that covers a range of (separately priced) web services from Amazon. The best known services are
There's quite a bit to AWS -- it's not a single product but a quite large suite of services. Costs vary depending on the service and your usage of it. Twitter has indeed used S3 in the past (not sure if they still do).