In articles, the purpose of headers and titles is to indicate what something is about. When I say headers, I mean short phrases (such as "Questions" and "Lists, images and source code" below) that are usually in a bold font and on a separate line within articles. When I say title, I mean the title of things like articles (the title of this article is "Headers and Titles for Articles"). Headers are the type of thing found in Tables of Contents in books and large articles.
Headers and titles should not be something that must be read. They should be something that helps to understand what is said in the relevant material and to help find the material later. There are exceptions, however. In a Frequently Asked Questions (FAQs) article each question is like a header. In an interview each question is like a header. Other than things like that, headers and titles should say nothing that must be read and should not be questions. People should be able to read an entire article without ever reading the headings. Headings are intended to help people find something in an article or to get an idea about what is being said, but the reader should be able to ignore the header.
Questions and sentences
I constantly see headers that are questions. When you look at the MSDN and read technical books, do you see headers that are questions? Very few, probably none, except as desribed previously. Since readers should be able to read an article without reading the headings, it makes no sense to make a heading or title a question.
Headers should not be complete sentences, except as desribed previously.
Colons
Headers should not end in a colon, as in "Introduction:".
Lists, images and source code
Things like lists, images and source code should have a description preceding them and not just a header. For source code, there should be a description of what it does and/or how it is used. Lists and images should have a description of what they are describing or whatever. The header, if any, should not be a complete description; the description of lists, images and source code should be more useful than any header that precedes them.
Titles
A title of an article should be as short and meaningful as possible. Sometimes that is difficult to do, since those two requirements (short and meaningful) can contradict each other. If we make a title short then it might be too vague of a description of what the article is about. Conversely, we often try to explain in a title everything that an article is about, but that can make the title long. Balancing the two takes a little time and consideration. Note that making a title a question does not help to satisfy either of those requirements. Sometimes I see a question mark after a title that is a good title and that is not a question, such as "How to do something".
Updates
| Date | Description |
| July 7, 2015 | Added that headers should not be a complete sentence and added that headers should not end in a colon. |