Web design contains many of the principles of print related graphic design such as typography, general visual aesthetic, color theory, placement of elements in order to lead the eye to a place of interest, breaking up information so that is is easily digestible, and many other principles that go into creating effective design.
So, let's say I understand the programming and I am an awesome designer -- what else should I know in order to create great websites as a designer? The next thing is structure, or architecture. This is a the foundation and grid of your layout, and there are specific grids that are proven to be effective in the design of websites. It also refers to the placement of elements on the screen, and the placement and proximity of these elements will either drive people through your site to deeper areas of information or drive them right off your website and on to one of the other websites that might have similar content but are easier to navigate and find the information or service that they are looking for. This is the reason that the logical stricture and architectures of your web design be done right -- simply because there are so many other websites out there and it is a lot easier for someone to click to another site than it is for someone to put down the print-related graphically designed communication they are reading or looking at. The amount of choices and quick ability to abandon a website make this factor of great importance. Architecture also refers to the navigation structure and how pieces of information are logically broken down into digestible pieces for the website visitor. It is a necessity for a web designer to be able to create visually pleasing designs as well as formulate a logical structure -- this is easier said than done. Visual or creative people tend to be less concerned with structure and logic, which is why a web designer requires a special trait -- they need to balance the two often opposing sides, or the right-brain and left-brain in order to create effective designs for the web.
So far we have awesome graphically and visually appealing design, an intimate understanding of the programming underneath the designs and we are able to balance our design into a logical, clear and concise structure so people can find what they are looking for, when they want it! What else do we need as an effective web designer? Let's call this one "on-page strategy". On-page strategy will enhance the user experience by combining the above three principles into something called strategy. Web design strategy depends greatly on a very good "discovery" period -- this looks at the competitor landscape, it reviews the main objective of the website as well as the secondary and tertiary objectives of the business and the website. On page strategy also includes a good SEO (search engine optimization) plan. SEO entails a good coding structure so that your valuable web design copy can be deciphered not only by the humans that read your website, but also by the search engines, so that they can rank your information and index it on their search engines -- and in turn making the vast amount of people able to find your website. It examines the information and makes decisions based on this information.
Then we have "off-page strategy" which refers to how other web sites will lead people into your website -- sort of like road signs that take people straight on to the street that your site lives. Some off page strategy includes advertising and link building, good public relations, article writing, web sit submission to directories and of course social media.
So, in summary web design requires a whole new set of skills and an expansion of the palette of a print-related art director or graphic designer. Throw in some motion design experience, video experience, maybe a dash of 3D and the art of simplicity and you're well on the road to designing great website experiences that are effective!