Many people use both these terms interchangeably. But there is an important difference.
Web design is all about the front-end and visual elements of the website and creating a great first impression for customers. It is the layout and design of the website. It can include anything from the wireframe, mockup, color scheme and content.
Web designers use different design tools such as Adobe Photoshop, Adobe Illustrator to create the layout and other visual elements of the website.
Web development, on the other hand, includes all the code/programming that takes place on the backend of the website. This includes the coding of all the designed pages, applications, and functions that empower your website.
Web developers use HTML, CSS, Javascript, JQuery, PHP and other different programming languages to bring to life the design mockup.