Website Design is essentially the process of sitting down, working out what you require for your website, and then designing the visual and graphical look of the site. Building a website involves two main stages, and website design is the 1st of these.
All other consequent stages of building website will be base upon website design: website structure, web pages appearance, functionality and so on, making it very important website development phase.
What you are left with after web design is one large picture of how your website should look. Turning this picture into a working website is the 2nd stage- web development or website development.