Semantics and Structure elements

<article> container box for text content
<aside> container box that represents content related to but physically separated from the main content (footnotes, pull quotes?)
<bdi> isolates a span of text that might be formatted in a different direction from other text outside it
<details> hidden container that, upon request, shows more information about an article or other
<dialog> used to define a dialog box or a window
<figcaption> describes the content of a element; used as a text descriptor
<figure> specifies the container where we want to place images, illustrations, drawings, etc?
<footer> container box for useful information about the parent container (not necessarily the document itself)
<header> container box for headings and other header based contents, such as search form or company's logo
<hgroup> used to represent headings of a section
<main> represents a container with the main content of the body element (the content wrapped by <main> tag has to be unique
<mark> indicates a text that we want to highlight
<menu> defines a list of commands or menu items that may be applied as context menus, toolbars, etc...
<menuitem> defines a single command or menu item that will be applied as a part of the <menu> element
<nav> describes a set of navigational links
<rp> used in ruby annotations for the benefit of browsers that don't support Ruby annotations (child element to <ruby>)
<rt> contains the annotation that describes and explanation or pronunciation of Ruby annotated characters (child element to <ruby>)
<ruby> specifies a Ruby based annotation which is a text is a short annotation of some base text
<section> container box representing thematically grouped sections
<summary> represents a caption or a title of the <details> element
<time> represents a time on a 24 hour clock or a date in the calendar
<wbr> represents a position within text where the browser may optionally break a line

Graphic elements

<canvas> container box used to draw graphics in a HTML document (requires a script such as JavaScript to be functional)
<svg> container box for SVG based graphics (vector graphics in XML format)

Media elements

<audio> allows embedding audio files or streams directly into the HTML document
<embed> embeds a plugin or other external application into the documents
<source> specifies multiple media resources for other media elements (video or audio)
<track> specifies explicit external timed text tracks for other media elements (audio or video)
<video> allows embedding audio files or streams directly into the HTML

Form elements

<datalist> represents a predefined list of options for form controls, specifically for an <input> element
<keygen> specifies key-pair generated in public-private mode, where private key is stored locally and public in the server
<output> shows the result of a calculation or a user action

Progress and meter elements

<meter> used to measure data within a range
<progress> used to project a progress of a task

Math elements

<math> part of HTML MathML object that may be used to represent mathematical symbols and formulas