When building your website, design, SEO (search engine optimization), and usability should all overlap in your coding and design. You should build the website to target the demographic audience that you wish to reach.
Large companies sometimes spend in the millions for companies to obtain marketing data to increase their target audience by making sure they are reaching the maximum segment that will be using their product or service.
Design = Coding or graphics presented in HTML or PHP that describes the website to the visitors browser.
SEO = How well that website performs in search engine results.
Usability = The best path for the visitor to use the website.