All html tags definte structure and give meaning to a Website. HTML does not focus on presentation (style). Presentation is handled by CSS
Include html, head, title, and body. These tags definte the structure of an HTML page. You can't see them on a website.
Include em, strong, h1, h2, h3, h4, h5, and h6. These tags change the look of the page, but more importantly, give meaning to page items. CSS will be used to style the page (change the presentation), not the html.
Include, ol, ul, li, table, tr, and td. These tags help change how the information is presented on the page, with emphasis on meaning.