There is major difference between website designing and web site development. Though both are equally challenging but web site development is more sophisticated. Web site designing basically deals with look and feel of the website. But web development is the backend that runs the web applications.
Web applicationdevelopment comes with many challenges; the developer has to test it for multiple browser compatibility (thanks to browser evolution and the plugin support), cross platform compatibility (Windows, Linux, MAC, etc.), web security (as there are many waiting to hack your website), multi user support (you have no control of how many users access your website), graphics loading time and compatibility issues, etc. But the major challenge is security as the users (especially for e-commerce) share their confidential data over the internet with a belief of security. Big organizations have their own team of developers but not every organization can afford a team of web developers.
There are organizations that offer professional web development services and web applications development services; they perform coding as per the client’s needs and requirements. Not every client needs the secure data transfer but they do need data, browser compatibility, less load time etc. There are web applications especially designed for the clients with the right pricing.