This is a question without a single answer as it encapsulates many skilled and specialist areas of input. The web aspect is probably well known by everyone but the Magento web design part could elicit twenty different answers if twenty people were asked to define design.
The electronic dictionary gives seventeen different explanations for Magento design and beyond these definitions there are also different terms used by design teams working on Magento web pages. They individually all specialise in specific areas of the design, so the Web Designer who is a jack of all trades has become outdated and would only be a cog in the wheel of large Web Design Companies, who require many skills to complete a design.
In reality there are certain areas of a Magento website that can be designed such as the User Interface. This includes features like the navigation which dictates how quickly and smoothly a user can move along the different site paths and information architecture to create site structure and classification. This element involves developing the intellectual perception of the website structure and how users view the website.
Other vital elements are the visual aspect of a Magento website, where image and user feel for the site create a particular impression. This area of design includes graphics, layout, colour and styles and is governed by the elements and principles of design. Another important area of design is the structure of the site and how it is built. This incorporates the Magento coders or programmers marrying the visual elements, layout and user interface with the coding languages such as CSS, Java Script, HTML, PHP and C+. This process puts flesh on the skeleton and is basically the operations platform for activating the other design areas.
We now talk in terms of web design teams as it involves different types of designers who do different things. They are comprised of graphic designers or artists, information architects, electronic marketing professionals, user interface technologists and language coders. We can appreciate then that website design covers a collection of specialist skilled areas and that large projects in particular require a team approach as opposed to the nominal lone Magento web designer.