There is a fine overlap between a web designer and a web developer, but technically both are different professions and have important roles in the implementation of a fully functional website. As a client you should know the differences and determine when you need to hire a designer or a developer.
Web Developers on the other hand are in charge of the inner workings of a website and manage other more robust programming languages such as PHP, Python, Ruby, C#, etc. They work on top of the designer's framework and provide the technical input to make a website highly functional and dynamic.
Very seldom you can find someone who have both capabilities, but there are some people out there who do both very well. Designers are more artistic and developers are more analytic, meaning that both use a different side of their brain to carry out their functions. But again, there are exceptions to every rule and you may be lucky enough to meet someone who manages both roles.
With the advent of open source projects, such as WordPress, Joomla and Drupal, there has been a hybrid approach to web development and design because now everything is focused on a very structured environment. What this means is that designers can work around a particular framework designing specific themes that are fully functional and well integrated with standard parameters. Also, web developers can program small applications to enhance the basic framework and complement the design around it. This hybrid approach and the integration of powerful Content Management Systems' engines has made it possible to implement very complex websites is record time with a 99.9% of bullet-proof functionality.
At Azulia we work with dedicated teams of professionals in the area of web design and web development and we provide expertise in the integration and management of the most widely used content management systems: WordPress, Joomla and Drupal.