Codex

Interested in functions, hooks, classes, or methods? Check out the new WordPress Code Reference!

Templates

Templates are the files which control how your WordPress site will be displayed on the Web. These files draw information from your WordPress MySQL database and generate the HTML code which is sent to the web browser. Through its powerful Theme system, WordPress allows you to define as few or as many Templates as you like all under one Theme. Each of these Template files can be configured for use under specific situations. More information on creating Themes can be found at Theme Development.

General Theme Articles

Using Themes 
Basic information about using and installing themes and templates
Migrating Plugins and Themes 
Content information how to upgrade a theme for compatibility with new versions of WordPress
Theme Development 
How to develop and build your own themes
Theme Frameworks 
Build a child theme on the foundation of a parent theme. You can start with a theme that you like and easily modify the parts you want to change. Do more in less time.
Theme Review 
How to prepare your Theme for public release and submission to Theme Directory
I18n for WordPress Developers 
Internationalization, including a section on how to internationalize your theme
Theme Compatibility List 
List of which Themes are compatible with various Versions WordPress
Creating Admin Themes 
How to create a plugin that modifies the look of the Admin section
Integrating WordPress with Your Website 
Using WordPress with your existing website

Template File Articles

General References

Stepping Into Templates 
Introduction to the building blocks of WordPress Themes, the template files. Explains how they work together to build a web page and how template files can be included in other template files.
Template Hierarchy 
Description of the order of preference of templates for the generation of various pages. Briefly lists the various templates that WordPress checks for in the process of generating a requested page on the weblog.
The Loop 
Description of the WordPress Loop, which is the part of a template file that generates the content
The Loop in Action 
More about the WordPress Loop
WP Query 
WP Query is a class that refers to the $wp_query object used in the WordPress Loop. There are times when you may have to interact with the WP Query, for instance when creating multiple Loops, or when dealing with Conditional_Tags.

Header, Footer, Sidebar, and Content Sections

Designing Headers 
Customizing the header.php template file.
Adding Post Feeds to the Header 
How to add RSS feed links to your header template file
Customizing Your Sidebar 
Customizing the sidebar.php template file.
Customizing the Read More 
Customizing the look of Read More links on your excerpts
Next and Previous Links 
Customizing the look of next/previous post/page links
Styling Page-Links 
How to break up single posts into multiple pages, and customize their links
Separating Categories 
Putting custom separators in the category list for your posts
Using Custom Fields 
How to add custom meta-data to your blog posts, and how to display it
Adding Asides 
How to add side notes to your blog posts
Enabling Popup Comments
How to make comments come in a pop-up window

Archives, Categories, and Special Pages

Creating a Static Front Page 
How to make a static home page, or splash page, for your blog
Author Templates 
Customizing the author.php template file which showcases information about the author and their posts.
Category Templates 
All about defining and customizing templates for the various category archives for your blog.
Tag Templates 
All about defining and customizing templates for the various tag archives for your blog.
Alphabetizing Posts 
How to make a category page use alphabetical rather than chronological order
Creating an Archive Index 
Customizing the archives.php template file which showcases posts by date.
Pages 
Describes WordPress Pages, and the relationship between Templates and Pages feature.
Page Templates 
Describes the Template Hierarchy for Pages and Custom Page Templates.
Styling Theme Forms 
Describes how to make forms in your templates for comments, search, etc.
Creating a Search Page 
Customizing the search.php template file.
Creating an Error 404 Page 
Creating a template file Page to feature a customized Error 404 Page.
Displaying Posts Using a Custom Select Query
Modifying which posts are displayed on a page
External Resources

Template Tags Articles

Stepping Into Template Tags 
Introduction to the use of template tags in template files.
Template Tags 
The core set of functions used to draw information from the database and display it on the web. For example, the Content of a Post is displayed with the Template Tag the_content().
Anatomy of a Template Tag 
Understanding the structure, usage, and parameters of template tags used in template files.
Include Tags 
A document describing the tags that are available to include and use WordPress' template files.
Conditional Tags 
These tags (always of the form is_*(), such as is_home()), report what sort of page is being displayed (e.g. the Main Page, a Category Archive, etc.) and can be used within a Template to control the Template's output depending on what the tags report.
Query Posts Template Tag 
A description of the query_posts template tag, which is used to control which posts are displayed on a page. Query posts alters the main query that WordPress uses, and is therefore not recommended to display different posts than those that would normal show up at a specific URL. It is best to use pre_get_posts for any changes that need to be made to the main query.
Get Posts Template Tag 
A description of the get_posts tag, which is used to retrieve a list of posts matching criteria set through the tags parameters. This is the preferred method for quickly creating new criteria for your Loop, or to create a specific array of posts.

CSS, Design, and Layout

CSS 
How to integrate CSS into WordPress, with list of other WordPress CSS references
Blog Design and Layout 
List of articles and resources for WordPress blog design, and layout
Know Your Sources#CSS 
List of general CSS references

Tools, Testing and Validating

CSS Fixing_Browser Bugs
CSS Troubleshooting
Validating a Website
Plain Text Editors 
A list of plain text editors for all platforms.