Check out this excellent resource, the Application templates for Windows SharePoint Service 3.0 page on the Microsoft TechNet website.
Application templates are out-of-the-box custom scenarios tailored to address the needs and requirements of specific business processes or sets of tasks in organizations of any size. They can be applied to a task, such as managing a help desk or tracking a marketing campaign, to easily create a dedicated Web-based application for a more efficient and effective way of working.
While application templates can be used to solve particular business needs, they also provide a starting point for partners and developers looking to build deeper SharePoint-based solutions. The templates make use of Windows SharePoint Services 3.0 capabilities and are compatible with Microsoft Office SharePoint Designer 2007 to help make customization easier.
Application Templates for Windows SharePoint Services 3.0 are separated into two groups, site admin templates and server admin templates, providing the right combination of integration with the platform and ease-of-deployment within an organization.
Site admin templates are custom templates that are easy for any SharePoint site administrator to install into the template gallery without requiring server administration access.
Server admin templates were created as site definitions, enabling tighter integration and enhanced functionality within the Windows SharePoint Services platform. They require a server administrator to install.