The lists below are compiled based on data at W3Schools, comparing HTML 5 with HTML 4, not XHTML 1.
New tags
Tags you should learn.
<article> Defines an article
<aside> Defines content aside from the page content
<audio> Defines sound content
<canvas> Define graphics
<command> Defines a command button
<datagrid> Defines data in a tree-list
<datalist> Defines a dropdown list
<datatemplate> Defines a data template
<details> Defines details of an element
<dialog> Defines a dialog (conversation)
<embed> Defines external interactive content or plugin
<event-source> Defines a target for events sent by a server
<figure> Defines a group of media content, and their caption
<footer> Defines a footer for a section or page
<header> Defines a header for a section or page
<mark> Defines marked text
<meter> Defines measurement within a predefined range
<nav> Defines navigation links
<nest> Defines a nestingpoint in a datatemplate
<output> Defines some types of output
<progress> Defines progress of a task of any kind
<rule> Defines the rules for updating a template
<section> Defines a section
<source> Defines media resources
<time> Defines a date/time
<video> Defines a video
<xmp> Not supported. Defines preformatted text
Deprecated tags
Do not use them anymore!
<acronym> Not supported. Defines an acronym
<applet> Not supported. Defines an applet
<basefont> Not supported. Use CSS instead
<big> Not supported. Defines big text
<center> Not supported. Defines centered text
<dir> Not supported. Defines a directory list
<font> Deprecated. Defines text font, size, and color
<frame> Not supported. Defines a sub window (a frame)
<frameset> Not supported. Defines a set of frames
<isindex> Not supported. Defines a single-line input field
<noframes> Not supported. Defines a noframe section
<s> Not supported. Defines strikethrough text
<strike> Not supported. Defines strikethrough text
<tt> Not supported. Defines teletype text
<u> Not supported. Defines underlined text