Amazon has a very rich and deep IDM ecosystem. There are multiple entry points into the Amazon world of identity for enterprises that are interested in using features of Amazon.
Amazon Web Service (AWS) the set of tools to work within Amazon’s cloud platform offers integration points for SAML and ActiveDirecotry to name a few. These two integration points enable leveraging your current enterprise ID’s for use of and by various services within the Amazon AWS tier of services.
To use Amazon as an external ID provider and thus provide access inside your enterprise application is an entirely different realm of IDM at Amazon. This service is called “login.amazon.com”. Using this service is similar to a login with Facebook experience from the end user perspective. This is mainly targeted at sellers and online businesses but flexible enough to use as a generic provider as well. The added hooks with using your Amazon ID are the ability to "pay with Amazon".