What are the differences between extending a web app and adding a host header then adding alternate access mappings in SharePoint 2010? My understanding is that , if you are using different authentication scheme for different zones , then extending a web app. If you would like to access the same web apps but with different URL for different zones ,using adding a host header then adding alternate access mappings. Please feel free to correct me if I got it complete wrong.
Extending the application allows you to view over a number of different hosts (using AAM) but more importantly allows you to specify a different authentication provider and security settings.
See the following answered question for more info on this.
SPDoctor has a very good explanation on the difference between the 2 methods for managing IIS and host headers for your SharePoint server. It is always best to let SharePoint handling IIS by extending the web application instead of manually creating host headers and alternate access mappings.