There are many different types of content on the iText website, for example landing pages, blog posts, tutorials, etc. All these different types of content have their own content type in the Drupal Content Management System, which defines various default settings for that type, such as whether it is published automatically and whether comments are permitted. It also provides a basic structure for the specific content, for example it specifies that the Blog Post content type has to have a title, a body of text, a target site (customer area, developer area or both) and tags. You can see what elements are needed for each content type on their content editing forms.
The iText website also uses a set of content building blocks: Paragraphs. Paragraphs in this sense are different from paragraphs in written text, they are predefined layouts for chunks of content. For example a Paragraph can have an image on the left and some text on the right, while another one has three boxes in a row where you can list specified content. Paragraphs are used to build Landing Pages on the iText website.
A Landing Page is a web page that appears in response to clicking on a search engine optimized search result or an online advertisement, but its URL can be also shared through posters, flyers or social media. On the iText website, Landing Page is a flexible content type that you can use to build product pages, and other information pages (e.g. about us, use cases, etc.).
A book is a set of pages tied together in a hierarchical sequence, perhaps with chapters, sections, subsections, and so on. You can use books for manuals, site resource guides, technical documentation, or whatever you'd like. Books on the iText website can be exported into Leanpub format.