The main tool of a web designer is a graphics program such as photoshop. They do not necessarily need to know any HTML etc as they can easily get a coder to write that for them. Good web design is definitely not easy. You need a really good knowledge of colour theory as well as knowing a lot about accessibility and usability.
A web developer will know server side languages and probably HTML etc as well. It involves a completely different skill set and to get the best designed and functioning web site you need an expert from both areas - something almost never found in the one person.