Basic Tags
<!doctype html>
The document type declaration identifies the document as an HTML5 document
<html></html>
The entire document is within the html element. The html element is called the root element because it contains all the elements in the document. Within the html element, the document is divided into a head and a body.
<head></head>
The head element contains descriptive information about the document itself, such as its title, the style sheet(s) it uses, scripts, and other types of "meta" information
<body></body>
The body element contains everything that we want to show up in the browser window

Header Tags
<meta charset="UTF-8" />
This specifies the character encoding (the standardized collection of letters, numbers, and symbols) used in the document
<title></title>
Puts the name of the document in the title bar, helps search engines find your site, and if the name of bookmarked page

List Tags
<ol></ol>
Creates an ordered (numbered or alphabetical) list 
<li></li>
each numbered item in list
<ul></ul>
Creates an unordered (bullets only) list
<li></li>
each bulleted item in list
 
Text Tags
<pre></pre>
Creates preformatted text
<hl></hl>
Creates the largest headline
<h6></h6>
Creates the smallest headline
<em></em>
Emphasizes a word (italic)
<strong></strong>
Emphasizes a word (bold)

Links
<a href="URL"></a>
Creates a hyperlink
<a href="mailto:EMAIL"></a>
Creates a mailto link
<a name="NAME"></a>
Creates a target location within a document
<a href="#NAME"></a>
Links to that target location from elsewhere in the document

Formatting
<p></p>
Creates a new paragraph
<p align=?>
Aligns a paragraph to the left, right, or center
<br>
Inserts a line break
<blockquote> </blockquote>
Indents text from both sides
<div align=?>
A generic tag used to format large blocks of HTML, also used for stylesheets

Graphical Elements
<img src="name">
Adds an image
<img src="name" align=?>
Aligns an image: left, right, center; bottom, top, middle
<img src="name" border=?>
Sets size of border around an image
<hr>
Inserts a horizontal rule
<hr size=?>
Sets size (height) of rule
<hr width=?>
Sets width of rule, in percentage or absolute value
<hr noshade>
Creates a rule without a shadow

HTML5 Page Structure
<header></header>
The header element is used for introductory material that typically appears at the beginning a page or at the top of a section or article.
<footer></footer>
The footer element is used to indicate the type of information that typically comes at the end of a page or article
<aside></aside>
The aside element identifies content that is related but tangential to the surrounding content. Aside can be used for pull quotes, background information, lists of links, callouts, or anything else that might be related (but not critical to) a document.
<section></section>
The section element has a broad range of uses, from dividing a whole page into major sections or identifying thematic sections within a single article.
<article></article>
Use the article element for self-­-contained works that could stand-­-alone or be reused in a different context. Long articles can be broken up in different sections. If the content is self-­-contained and could appear outside the current context, it is best marked up as an article.
<nav></nav>
The new nav element gives developers a semantic way to identify navigation for a site.
<div></div>
A div element is used to create a logical grouping of content or elements on the page. It indicates that they belong together in some sort of conceptual unit or should be treated as a unit by CSS or JavaScript. A div element indicates a division of content. <div align="center">Whatever content you want centered on the page</div>

HTML5 and Internet Explorer
<!--[if lt IE 9]>
<script src="http://html5shiv.googlecode.com/svn/trunk/html5.js">
</script>
<![endif]-->

Include this code before the <body> element (i.e. in the <head>) but doesn't matter if it appears before or after the CSS -­- but for the sake of performance, it would make better sense to include the CSS first then this script.
Reference   HTML & HTML5 Cheatsheet