User Interface Design is the design of websites, computers, appliances, machines, mobile communication devices, and software applications with the focus on the user’s experience and interaction. The goal of user interface design is to make the user’s interaction as simple and efficient as possible, in terms of accomplishing user goals—what is often called user-centered design.
The back-end, or the "server-side", is basically how the site works, updates and changes. The user doesn’t see it or directly interact with it as with client-side technology, but it’s always running in the background, delivering smooth functionality, a desktop-like experience, and information from the database right into the browser. A dynamic site is a site that's constantly changing and updated in real-time.
An application program (app or application for short) is a computer program designed to perform a group of coordinated functions, tasks, or activities for the benefit of the user. An application include a word processor, a spreadsheet, an accounting application, a web browser, a media player, an aeronautical flight simulator, a console game or a photo editor. The collective noun application software refers to all applications collectively.