Web Design & Development are terms that are most often used to broadly describe the two main aspects of building websites.
Web Design is usually associated with the “Look & Feel” of the website. A Web Designer will work with graphics, images, colour and type to produce layouts that effectively communicate information to the user.
Web Development is best thought of as making the Design of the Web Designer a reality as well as implementing the functionality of the website. It can be as simple as creating the interactivity of a website’s main menu, or a contact form, but also more advanced features such as interacting with a database. The functionality of an e-commerce site is a good example, where users can purchase products and have their own customer accounts on the website. The products and customers of such websites are stored in a database. The Web Developer will ensure that the integration of the product & customer database with the website functions as intended in the most optimal way. A web developer may build an application from the ground up, or work with an existing platform such as WordPress.
Both Web Design & Development are broad terms that could be further broken down into more precise roles, depending on the scope of the project and the size of the production team. Web Design might include a Graphic Designer and a User Interface/Experience Designer. A Web Development team will often have a Front-end Developer for implementing the design & simple interactivity and will sometimes work with simple programming languages. For the more advanced functionality, a Back-End Developer is essential. They will almost always work with more advanced programming languages that interact with a database. Not all websites require a Back-End Developer, usually just those needing advanced features & functionality.
Strictly Speaking, Web Design & Development are not terms that define the entire process of creating a website. Marketing Specialists, SEO & Content Strategists, Copywriters and many more roles can be involved and this is usually determined by the scope and size of the websites. Web Agencies may perform these roles internally or outsource these tasks as needed.