PeopleSoft Fluid User Interface is one of the most significant feature in PeopleTools 8.54. It gives flexibility to a developer to dynamically change the PeopleSoft applications display based on user’s device type. As a developer you would still be using PeopleSoft Application Designer to create Fluid pages but there will be some difference when it comes to designing pages.
PeopleSoft Classic Pages can still be designed in the same way you have been doing it in past. PeopleSoft pages usually follow a standard format and are based on WYSIWYG (what you see is what you get) scenario. With PeopleSoft Fluid Pages, emphasis is more on using CSS and HTML5 to create a page which can dynamically change the appearance and position of the fields being used. You can also specify no. of fields & data that you want to display for different device types.