TL. DR.

  

What are these Levels A, AA, and AAA? What types of disability do   part of each group? What are the criteria for each of these levels?   What my site needs to meet to be at least a Level A of   accessibility?

There is a ranking of the websites provided by W3 (WCAG) based on how much your site meets, or can serve, the public, where A , AA and AAA are levels, the first being the less the relavante, and the latter the more. To understand the operation of the same it is necessary to understand that it is divided into 4 principles for the development of a website:   noticeable, operable, understandable and robust.

Perception

• Information and user interface components should be presentable to users in ways they can understand. Nielsen's heuristics deal with various points of perception, how easy it is to understand what each component of your site is.

Operable

• UI components and navigation must be operable (operational).

Understandable

• operation and information should be understandable.

Robust

• Should be robust enough information that a large majority of people (users) can interpret correctly, including assistive technologies.

Translated and adapted portion of the WCAG website .

Within each major principle there is a evaluation series , which at the end it will generate a note and tell you whether or not it will be a ranked site. Each Assessment is detailed by the site, it would be huge to put only one of them, so I will not explain every 1.

For an A there is a minimum of requirements to be met. These can be found here .

  

There are audit tools, or some Extension or   Browser DevTools that I can use to evaluate what the current level is   accessibility on my site?

W3 Provides a list of accessibility tools, all of which I have used are free, or a amount of use is. Thisexamplewastakenfromacheckmadeonthis site (listed in the WCAG). It shows a list detailing what can be improved and can be found in the list.

  

The accessibility and grade of this Level can be used by Google as   site ranking criterion? Can I get punished for having a website   little affordable?

First we need to understand the google algorithm and its search . Both links are in Portuguese, including the research includes an explanatory video (and very well explained) of the subject. The main punishment of a website without usability is just its "forgetfulness", that is, if it does not infringe any law or cause any misconception that could harm its user. Sites with more advertisements (on other sites) tend to have more indexes, therefore tend to stay ahead of others.

Note: All lists of improvements, information / technical instructions are available on the WCAG website.