The User Interface can be said as the look and feel experience of using that particular application or system. With regard to the mobile application, it is the way in which the application unfolds, the way in which the application responds to the user commands, the color palette and many more attributes.
The user interface previously present in the desktop, web applications or websites are starkly different from that of the user interface of a mobile application. The prime aspect is that the pace at which the trend changes with regard to the mobile application aspects. In the past the trend changed only when the market leaders change any particular aspect. But, not anymore. Now even the small scale developers or startups with a refreshing and a captivating idea gauge the trends in place.
Type of Sense you need in the application
Touch is the prime most sensory type, on which most Mobile applications are developed at present. Smart phones being the most common device used, touch is going to be the most used sense in near future too. Apart from the touch, gesture is also another sensory type used in some applications. But, the efficiency and precision of gesture control still holds it back.
Speech based UI is the other sense used by some applications at present. But, apart from the personal managers like the Siri rest of the application of speech based usage is finicky. The speech recognition engine still has a long way to go, and it will in fact will. But, at present it doesn’t have much utility.
Need to satisfy a mixed audience
An application in particular and mobile platform in general has novice audience at the beginning, and will gradually set to gain expert users along the line. This also suits for a particular user, who uses a smart phone for the first time and slowly gets to know the intricacies and the general norms on which the applications are based. An expert user prefers more efficiency, whereas a novice user being uncertain about certain functionality of the application menus and buttons wants the application to be easy-to-use. So it’s the developer’s responsibility to design the app in such a manner to cater everyone’s needs.
How much can be too much
Unlike websites or desktop software, the mobile application cannot be lengthier beyond a certain extent. So it should be delineated in such a manner so that the user should neither feel cramped in a single page, nor exhausted with minimal information in numerous pages.
Apart from these, the typography, graphic design, color palette and aesthetic appeal also matters while a UI is designed for a mobile application. Prepare a workflow, and a checklist before plunging into the UI design. It will save a lot of time in iterations and future corrections immediately after the application launch. An UI cannot be categorized as special simply with its design standards, its more about the suitability of that UI for that application which makes it special.