The Feature Layer
Feature modules are the functionality of the website such as navigation, search, content, accounts, etc. The bulk of the business logic and presentation elements sits in these modules.
The accounts module handles user accounts and user profiles including login, registration, forgot password and profile editing.
The Demo module contains a number of features used for demonstrating aspects of the Sitecore platform, such as triggering a campaign from external sites or information about what the Sitecore Experience Database contains.