Whay exactly does user interface mean?

The term 'user interface' referrs to the means through which a user operates software. When the term is qualified further with 'graphic', it referrs to the graphical elements that are combined or recombined into a page layout for a user to interact with software.

The context of 'user interface' in the job description pertains to the elements of a webpage that have a function to do with operating the site or data flow through the site. This would include menus, forms, links or any other means that a user interacts with the webpage.

UI Design incorporates many branches of human study into one, specialised field. The goal of which is to optimise the interface for given parameters. The parameters may include disabled usability, optimised production, sales generation or enhanced user experience and the list goes on. Typically an UI designer has a working knowledge of human psycology, cultural context of symbols and colours, semantics, and familiarity with existing UI projects, such as the Gnome project for example, to build upon.

A definition of a simple UI will always be coloured by the answerer's background and opinions. In my opinion, a simple UI will have little or no decorative graphical elements that distract from the functional elements. A simple interface would have a reduced number of possible actions that can be performed and only the absolute minimum number of elements required to perform the action on any given interface.

You only need to know about how a given back-end works in order to build an effective UI; you don't need a developer's knowledge of the back-end. Typically developers are the poorest UI designers as they cant see the trees for the forrest and loose touch with how a system appears to a end user.

I don't think you're going to find a few quick tutorials on the web to cover UI design. The topic is far too broad. I was introduced to UI design in my undergraduate degree in multimedia design and refined my knowledge with a masters degree in design. The bulk of the tutorials I come across on the web are overly focused on aesthetics rather than function. So I would recommend checking out tertiary coursework reading lists and go to the library or book shop and grab a few titles that are related to the field you're interested in.

The UI elements include the top nav, side nav and content area. The navs are interactive, the content is passive (excluding inline links). The header graphic and images down the side are fluff, or UI decoration. Well, calling the header graphic purely decorative is misleading; it does serve the purpose of naming the site but does nothing else.

A simple UI improvement for that site may be scaling back the header image to 1/3 height for content pages, site name goes to the left, site section name to right. This would give the header graphic more than one function. Another UI improvement is for the top nav buttons to change colours when the user is in that section. The ui feeback helps to orientate the user.