Digital marketing refers to advertising delivered through digital channels such as search engines, websites, social media, email, and mobile apps. Using these online media channels, digital marketing is the method by which companies endorse goods, services, and brands.
Web design refers to the design of websites that are displayed on the internet. It usually refers to the user experience aspects of website development rather than software development.
Affiliate marketing is the process of earning a commission by promoting other people's (or company's) products. You find a product you like, promote it to others and earn a piece of the profit for each sale that you make.