Today the terms Web Design and Web Development are practically interchangeable. But the truth is that they actually refer to two completely different services provided by web companies.
In a nutshell: Web Design refers to the aesthetic part of the site. Normally the task of Web Design is performed on Adobe Photoshop where the designer will plan the layout and create all the visual elements. On the other hand, Web Development takes the design and transforms it into a functioning site using code.
Web designers consider the client’s objective for the site and the audience that the website must reach. They are often the people who communicate with the client directly to negotiate the style of the site itself. They use basic principles to ensure the user interface is acceptable and useable.
Web Developers (often called programmers) take that design and build a website. A design is a non-interactive image of a site, a developer will make the design usable.