A layout includes the HTML for a basic webpage structure and allows you to provide your own content. This gives you more freedom over the appearance of your design. A theme includes three pages (index.html, about-us.html, and contact-us.html) that include the HTML and images for a basic design, allowing you to replace the images and text with your own content. This is a great option for beginners, since the bulk of the work has already been done for you.