Accessible Rich Internet Applications (ARIA) is a set of attributes that define ways to make Web content and Web applications (especially those developed with JavaScript) more accessible to people with disabilities.

It supplements HTML so that interactions and widgets commonly used in applications can be passed to Assistive Technologies when there is not otherwise a mechanism. For example, ARIA enables accessible navigation landmarks in HTML4, JavaScript widgets, form hints and error messages, live content updates, and more. 

Many of these widgets were later incorporated into HTML5, and developers should prefer using the correct semantic HTML element over using ARIA, if such an element exists. For instance, native elements have built-in keyboard accessibility, roles and states. However, if you use choose to use ARIA, you are responsible for mimicking (the equivalent) browser behaviour in script.

Here's the markup for a progress bar widget:

<div id="percent-loaded" role="progressbar" aria-valuenow="75" aria-valuemin="0" aria-valuemax="100" />

This progress bar is built using a <div>, which has no meaning. Unfortunately, there isn't a more semantic tag available to developers in HTML 4, so we need to include ARIA roles and properties. These are specified by adding attributes to the element. In this example, the role="progressbar" attribute informs the browser that this element is actually a JavaScript-powered progress bar widget. The aria-valuemin and aria-valuemax attributes specify the minimum and maximum values for the progress bar, and the aria-valuenow describes the current state of it and therefore must be kept updated with JavaScript.

Along with placing them directly in the markup, ARIA attributes can be added to the element and updated dynamically using JavaScript code like this:

// Find the progress bar <div> in the DOM.
var progressBar = document.getElementById("percent-loaded");

// Set its ARIA roles and states, so that assistive technologies know what kind of widget it is.
progressBar.setAttribute("role", "progressbar");
progressBar.setAttribute("aria-valuemin", 0);
progressBar.setAttribute("aria-valuemax", 100);

// Create a function that can be called at any time to update the value of the progress bar.
function updateProgress(percentComplete) {
  progressBar.setAttribute("aria-valuenow", percentComplete);
}

Note that ARIA was invented after HTML4, so does not validate in HTML4 or its XHTML variants. However, the accessibility gains it provides far outweighs technical invalidity.

In HTML5, all ARIA attributes validate. The new landmark elements (<main>, <header>, <nav> etc) have built-in ARIA roles, so there is no need to duplicate them.

Tutorials

Introduction to ARIA
A quick introduction to making dynamic content accessible with ARIA. See also the classic ARIA intro by Gez Lemon, from 2008.
Videos of Screen Readers Using ARIA
See both real and simplified examples from around the web, including "before" and "after" ARIA videos.
Using ARIA
A practical guide for developers. It suggests what ARIA attributes to use on HTML elements. Suggestions are based on implementation realities.

Simple ARIA Enhancements

Enhancing Page Navigation with ARIA Landmarks
A nice intro to using ARIA landmarks to improve web page navigation for screen reader users. See also, ARIA landmark implementation notes and examples on real sites (updated as of July 2011).
Improving Form Accessibility
ARIA is not just for dynamic content! Learn how to improve accessibility of HTML forms using additional ARIA attributes.

ARIA for Scripted Widgets

Writing keyboard-navigable JavaScript widgets
Built-in elements like <input>, <button> etc have built-in keyboard accessibility. If you 'fake' these with <div>s and ARIA, you must ensure your widgets are keyboard accessible.
Live regions
Live regions provide suggestions to screen readers about how to handle changes to the contents of a page.
Using ARIA Live Regions to Announce Content Changes
A quick summary of live regions, by the makers of JAWS screen reader software. Live regions are also supported by NVDA with Firefox, and VoiceOver with Safari.

Standardization Efforts

WAI-ARIA Specification
The W3C specification itself.
WAI-ARIA Authoring Practices

The official best practices documents how best to ARIA-ify common widgets and interactions. An excellent resource.

Videos

Following talks are a great way to understand ARIA:

ARIA, Accessibility APIs and coding like you give a damn! – Léonie Watson

Filing Bugs

File ARIA bugs on browsers, screen readers, and JavaScript libraries.

Accessibility, AJAX, JavaScript