All html tags define 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 define 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 presentaton), not html.
include ol, ul, li, table, tr, and td. These tags help change how information is presented on the page, with emphasis on meaning.
Again, HTML is meant to display information in black in white text. CSS is meant to style your information so it looks good.