In HTML 4 there were only two main content models, block and inline level elements. Block level elements would stack on top of each other in normal document flow while inline level elements typically appear within the flow of text content.
In this example the heading 1 and the paragraphs are block level items, while the bold tags and the link below are examples of inline level elements.
In HTML5 new content models have been created to expand the structure and semantic capabilities of HTML. There are seven main models: Flow, Metadata, Embedded, Interactive, Heading, Phrasing, and Sectioning. To learn more about them, visit the interactive graphic contained in the W3C HTML5 specfication.