Skilled in graphic design and working in a program like Adobe XD, web designers will create a visual template for the client to approve that resembles what will be published on their website. After a client has approved the designer's templates, they are then used by a web developer as the basis to start their coding process.
Developers work on the back end of your website, turning the web designers designs into the living code that makes up a website. Developers are versed in many programming languages and focus on the technical interactions, logic, and databases of a website, making a beautiful design function flawlessly.
Think of the people building your website as a brain, where the left hemisphere is creative and the right hemisphere is analytical - you need both hemispheres firing to create something that looks awesome and works a treat. Designers supply the creative, developers make it happen; both work with each other to push better UX design and build websites that please the user and help propel your business objectives.