A designer designs the web pages and a developer develops or implement the designed structure into usable form.
A web designer, typically only does the graphical part of a website, they make the layout, choose the colors, design the look and feel.
A web developer, doesn’t always understand color theory or design and things like that, their primary role is to write the code that makes everything happen, for example, when a button is clicked it might send some data to a server, your web developer would be responsible for writing the the web page code for this to happen.
That said, many designers these days are also web developers, and many developers are also designers, the job role has become more and more intertwined over the years.
Hope this helps!
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