web designer is a graphic artist who develops and styles objects for the Internet. Website designers are able to tap into their creative energy and design the overall layout and look for websites.
A web developer is one that develops applications and functionality for the Internet. It is rare to find a web developer that knows every programming language; web developers usually just focus on a few languages. Typically one developer will focus on the front end programming of a site using HTML, CSS and JavaScript while another developer will focus on server side programming like PHP, Java, Ruby, and .NET.