You must first decide if your application will support user interactions and, if so, which interaction model to use.
You might choose the Web Application or Portlet Application model when moving a Web application or portlet from the server to the client to reduce development and training costs. You might choose the Rich Client Application model when you require more control over the user experience.
If your application requires a user interface, then you must consider the device characteristics in the design of the user interface. For example, if you are developing a Web application for PDAs and laptops, then you can design the layout of the Web pages to fit within the constraints of the PDA screen size. When you run the application on the PDA, the Web browser will render the markup for the PDA screen size. When you run the same Web application on a laptop, the Web browser will render the same markup, which should fit within the larger screen size. Of course, there might be cost vs. usability tradeoffs in supporting a common user interface across multiple device types instead of tailoring the user interface for each device type. For example, multiple Web pages might be required to perform a set of related business operations on the smaller screen size of a PDA while a single Web page might suffice for the larger screen of a laptop.
Parent topic: Application design considerations