WCAG Levels A, AA and AAA: What's the Difference?

The short answer
A, AA and AAA are the three conformance levels in the Web Content Accessibility Guidelines (WCAG). Level A is the minimum. Level AA is the standard most laws and policies require. Level AAA is the highest level.

Each level is a set of specific success criteria, and each higher level includes everything below it.

How the levels fit together
First, we must acknowledge that the levels stack. To claim AA, a page must meet every Level A criterion as well as every Level AA criterion. To claim AAA, it must meet all three sets, this means the website must meet Level A, Level AA and Level AAA.

WCAG 2.2 has 86 active success criteria:

Level A: 31 criteria
Level AA: 24 more (55 total for an AA claim)
Level AAA: 31 more (86 total for an AAA claim)
A few rules decide whether a claim holds up:

It's all or nothing. Missing one AA criterion means the page doesn't conform at AA. There's no "mostly AA."
Whole pages only. You can't claim conformance for part of a page.
Levels are about impact, not difficulty. Level A covers barriers that block some people completely. AA covers barriers that make content hard to use. AAA adds extra support for specific needs.
Level A: the baseline
Level A removes the barriers that lock people out entirely. If a site fails Level A, some people can't use it at all.

Examples of Level A criteria:

Text alternatives (1.1.1). Images that carry meaning have alt text. A screen reader user can't learn what a chart shows without it.
Keyboard (2.1.1). Everything works with a keyboard alone. Someone who can't use a mouse can still open menus and submit forms.
Info and relationships (1.3.1). Headings, lists, tables and form labels are coded, not just styled. Assistive tech can then announce the structure.
Use of color (1.4.1). Color isn't the only way to show meaning. An error field also gets an icon or a message, not just a red border.
Page titled (2.4.2). Each page has a title that says what it is.
Level A alone isn't enough for most real-world users. It's the floor, not the target.

Level AA: the practical target
Level AA is the level nearly every law, policy and contract asks for. It removes barriers that don't fully block people but make content hard or tiring to use.

Examples of Level AA criteria:

Contrast (1.4.3). Regular text has a contrast ratio of at least 4.5:1 against its background. Large text needs 3:1.
Non-text contrast (1.4.11). Buttons, input borders and focus indicators have at least 3:1 contrast.
Resize text (1.4.4). Text can zoom to 200% without breaking the page.
Reflow (1.4.10). Content fits a 320-pixel-wide screen without scrolling sideways.
Focus visible (2.4.7). Keyboard users can always see where they are on the page.
Captions, live (1.2.4). Live video and audio have captions.
Target size, minimum (2.5.8). Click and tap targets are at least 24 by 24 CSS pixels, or have enough space around them. New in WCAG 2.2.
Accessible authentication, minimum (3.3.8). Logging in doesn't depend on remembering or solving a puzzle unless there's a way around it, like allowing paste or a password manager. New in WCAG 2.2.
AA is achievable for almost any site with good design and development habits. That's why it's the standard.

Level AAA: enhanced support
Level AAA adds the highest level of support for specific needs. The W3C itself doesn't recommend requiring AAA for entire sites. Some content simply can't meet every AAA criterion.

Examples of Level AAA criteria:

Enhanced contrast (1.4.6). Regular text has at least 7:1 contrast. Large text needs 4.5:1.
Sign language (1.2.6). Prerecorded video includes sign language interpretation.
Reading level (3.1.5). When text needs more than a lower-secondary reading level, a simpler version or summary is available.
Unusual words (3.1.3). Jargon and idioms are defined.
No timing (2.2.3). Nothing has a time limit, except live events.
Target size, enhanced (2.5.5). Targets are at least 44 by 44 CSS pixels.
Here's why AAA site-wide isn't realistic. A live auction must have time limits, which fails 2.2.3. A medical research article can't always be rewritten at a lower-secondary reading level without losing meaning.

Still, AAA criteria are worth meeting where you can. Plain language, extra contrast and bigger targets help many people, including people with learning and thinking differences.

What to aim for
Target WCAG 2.2 Level AA across your whole site, then add AAA criteria where they help your audience most.

Fix Level A failures first. They block people completely.
Work through Level AA until every page and process conforms.
Test with automated tools, manual checks, assistive tech and real users.
Publish an accessibility statement that names the WCAG version and level, and lists known gaps honestly.