Quite interesting feature in Sharepoint using AAM. Isn't it?
I just need one liner to logically and physically understand the same.
AAm is a web app level setting, it give you flexiblity to map Public Url to Internal Urls. We can configure 5 public urls in different zone( default,intranet,internet,extranet,custom).
Alternate access mappings enable a Web application that receives a request for an internal URL, in one of the five authentication zones, to return pages that contain links to the public URL for the zone. You can associate a Web application with a collection of mappings between internal and public URLs. Internal refers to the URL of a Web request as it is received by Windows SharePoint Services 3.0. Public refers to the URL of an externally accessible Web site.
Multiple internal URLs can be associated with a single public URL. Mapping collections can contain up to five authentication zones, but each zone can only have a single public URL. Mapping collections correspond to the following authentication zones:
AAM tells SharePoint how to map web requests (for example, browsing to the homepage of a SharePoint site) to the correct web application and site so that SharePoint can serve the correct content back to you. It then tells SharePoint what URL the users should be taken to as they interact with SharePoint.