ArcGIS Server Cloud Builder on Amazon Web Services uses a set of predefined Amazon Machine Images (AMI) when it builds a site. If you want to use an image with your own set of services and applications, you can build a site and preserve it as a template. You can then use the template to launch future sites, saving you some configuration work on each site.
Templates preserve the licensing of your site. For example, you cannot use a template created with an ArcGIS Server workgroup edition license to launch a site with more than one ArcGIS Server machine. Similarly, templates preserve the geodatabase configuration. If your template does not include a geodatabase running on its own instance, you cannot choose to include one when you create a site from the template.
The template is stored using Amazon Web Services and incurs a storage fee. You will see charges from both Amazon EC2 and Amazon S3 that pertain to the template.
Cloud Builder allows you to share your templates with users of other Amazon accounts. Likewise, you can import templates into Cloud Builder that other Amazon account owners have shared with you. The ability to share templates is useful if you are building a custom solution for a client, if there are multiple Amazon accounts in your organization, or if you have to set up a lab or classroom wherein each participant will use their own Amazon account.