A featured article exemplifies Wikipedia's very best work and is distinguished by professional standards of writing, presentation, and sourcing. In addition to meeting the policies regarding content for all Wikipedia articles, it has the following attributes.
well-written: its prose is engaging and of a professional standard;
comprehensive: it neglects no major facts or details and places the subject in context;
a lead: a concise lead section that summarizes the topic and prepares the reader for the detail in the subsequent sections;
appropriate structure: a substantial but not overwhelming system of hierarchical section headings; and
consistent citations: where required by criterion 1c, consistently formatted inline citations using footnotes—see citing sources for suggestions on formatting references. Citation templates are not required.
Prose. It features professional standards of writing.
Lead. It has an engaging lead that introduces the subject and defines the scope and inclusion criteria.
(a) It comprehensively covers the defined scope, providing at least all of the major items and, where practical, a complete set of items; where appropriate, it has annotations that provide useful and appropriate information about the items.
(c) In length and/or topic, it meets all of the requirements for stand-alone lists; does not violate the content-forking guideline, does not largely duplicate material from another article, and could not reasonably be included as part of a related article.
Structure. It is easy to navigate and includes, where helpful, section headings and table sort facilities.
The article is well organized and essentially complete, having been examined by impartial reviewers from a WikiProject or elsewhere. Good article status is not a requirement for A-Class.
More detailed criteria
The article meets the A-Class criteria:
Provides a well-written, clear and complete description of the topic, as described in Wikipedia:Article development. It should be of a length suitable for the subject, appropriately structured, and be well referenced by a broad array of reliable sources. It should be well illustrated, with no copyright problems. Only minor style issues and other details need to be addressed before submission as a featured article candidate. See the A-Class assessment departments of some of the larger WikiProjects (e.g. WikiProject Military history).
Very useful to readers. A fairly complete treatment of the subject. A non-expert in the subject would typically find nothing wanting.
Expert knowledge may be needed to tweak the article, and style problems may need solving. WP:Peer review may help.
The article reasonably covers the topic, and does not contain obvious omissions or inaccuracies. It contains a large proportion of the material necessary for an A-Class article, although some sections may need expansion, and some less important topics may be missing.
The article has a defined structure. Content should be organized into groups of related material, including a lead section and all the sections that can reasonably be included in an article of its kind.
The article is reasonably well-written. The prose contains no major grammatical errors and flows sensibly, but it does not need to be "brilliant". The Manual of Style does not need to be followed rigorously.
The article contains supporting materials where appropriate. Illustrations are encouraged, though not required. Diagrams, an infobox etc. should be included where they are relevant and useful to the content.
Readers are not left wanting, although the content may not be complete enough to satisfy a serious student or researcher.
A few aspects of content and style need to be addressed. Expert knowledge may be needed. The inclusion of supporting materials should be considered if practical, and the article checked for general compliance with the Manual of Style and related style guidelines.
The article is substantial but is still missing important content or contains much irrelevant material. The article should have some references to reliable sources, but may still have significant problems or require substantial cleanup.
More detailed criteria
The article cites more than one reliable source and is better developed in style, structure, and quality than Start-Class, but it fails one or more of the criteria for B-Class. It may have some gaps or missing elements; need editing for clarity, balance, or flow; or contain policy violations, such as bias or original research. Articles on fictional topics are likely to be marked as C-Class if they are written from an in-universe perspective. It is most likely that C-Class articles have a reasonable encyclopedic style.
Useful to a casual reader, but would not provide a complete picture for even a moderately detailed study.
Considerable editing is needed to close gaps in content and solve cleanup problems.
An article that is developing but still quite incomplete. It may or may not cite adequate reliable sources.
More detailed criteria
The article has a usable amount of good content but is weak in many areas. Quality of the prose may be distinctly unencyclopedic, and Wikipedia:Manual of Style compliance non-existent. The article should satisfy fundamental content policies, such as Wikipedia:Biographies of living persons. Frequently, the referencing is inadequate, although enough sources are usually provided to establish verifiability. No Start-Class article should be in any danger of being speedily deleted.
Provides some meaningful content, but most readers will need more.
Providing references to reliable sources should come first; the article also needs substantial improvement in content and organisation. Also improve the grammar, spelling, writing style and improve the jargon use.
A very basic description of the topic. Can be well-written, but may also have significant content issues.
More detailed criteria
The article is either a very short article or a rough collection of information that will need much work to become a meaningful article. It is usually very short; however, if the material is irrelevant or incomprehensible, an article of any length falls into this category. Although Stub-class articles are the lowest class of the normal classes, they are adequate enough to be an accepted article, though they do have risks of being dropped from being an article altogether.
Provides very little meaningful content; may be little more than a dictionary definition. Readers probably see insufficiently developed features of the topic and may not see how the features of the topic are significant.
Any editing or additional material can be helpful. The provision of meaningful content should be a priority. The best solution for a Stub-class Article to step up to a Start-class Article is to add in referenced reasons of why the topic is significant.