What a web designer does
A designer works out what each page is for, what goes on it and in what order, then designs how it looks on a phone, a tablet and a desktop. Good design is mostly about clarity: can a visitor tell within five seconds what you do, whether you serve them, and what to do next? Designers produce wireframes (rough layouts), mockups (the finished look) and a style system (colours, fonts, buttons) that the developer builds from.
What a web developer does
A developer turns the design into a working website. On a WordPress or Shopify project that means setting up the platform, building the theme or templates, configuring plugins or apps, connecting forms and payments, and making it load quickly. On a custom project it means writing the application itself. Developers also handle the technical side of SEO (structure, speed, markup), security, hosting and launch. More detail is in what a web developer does.
Which one do you need?
For a business website you need both, and you usually want them from the same team so nothing falls between the two. A beautiful design built badly is slow and breaks; a solid build with a poor design does not convert. When comparing providers, look at whether their past sites both look good and work well on a phone, and ask who does which part. The guide on finding the right web developer has the questions to ask.