Deciding Which to Use
At first you may be confused about the differences between page templates, declarative components, and bounded task flows and when you would want to use each.
You could use a page template when multiple pages have similar layout. For example, you may want to display several data elements with a master at top and detail at the bottom in a panel splitter. You may also want several pages to have a similar look and feel, such as branding, header, footer, or toolbars.
A declarative component is a composite of existing components that you define and reuse from the Component Palette. If you often find yourself constructing groupings of components that are the same, you can define a declarative component, so that all you need to do is drag it from the Component Palette to reuse it in multiple pages or applications. For example, you could define an address component that could be used whenever you display an address of any type in any application. That declarative component can be bound to different data, just like any component.
Bounded task flows are useful any time you have a set of activities and control flows that needs to be reused, such as a shopping cart or checkout in a shopping application, or creating an account in a banking application. You can call the bounded task flow. When constructed with page fragments, a bounded task flow can be used in a region on a page, so that the activities are displayed in the region without refreshing the entire page. You can create a portlet from a task flow as well.