Web design determines what the user does; web development determines what a website does. Web design focuses on the user experience, making sure visitors know what they need to do next and how to find things. Web development makes the features of a website that visitors use and experience. Without web development you end up with a static website that doesn't do what your users need it to do. Without web design you have a site that does great things, but looks and feels so bad that your users abandon it.