Web designers design how the website looks and feels.
They model the layout of the website, making sure it’s logical, user-friendly and pleasant to use.
They consider all the different visual elements such as color schemes and fonts - what buttons, drop-down menus and scrollbars should be included, and where; as well as what interactive touchpoints are needed to get from point A to B.
Web design also considers the information architecture of the website, establishing what content will be included and where it should be placed.
Web design is an extremely broad field, and will often be broken down into more specific roles such as User Experience Design, User Interface Design and Information Architecture.
In short, a web designer is the architect, while the web developer is the builder or engineer.