Effective Tags - Bring more attention to your question
Each tag should stand on its own: if a tag only makes sense when used in a group with other tags, it's wrong. For example, tagging a question as [visual][studio] (two tags) is wrong.
Pick tags that show higher counts in the look-ahead prompts, as they are more likely to make your question appear highlighted for a user on the main page or show up in someone's feed of watched tags.
At a minimum, try to include at least one very broad tag (e.g., java or c#) and one other tag to narrow the topic down within that broader category (e.g., strings or garbage-collection).
You are limited to five tags, and you are generally better off trying to use as many of them as you can, provided they follow the guidelines here.
Try to use broad tags. For example, you usually want to include both a general tag (windows) and a specific version tag (windows-10) rather than just the specific tag. While tagging a question cortana on Super User might convey exactly the information you intend (it implies Windows, Windows 10, and Cortana all in one tag), tagging it windows windows-10 cortana will bring your question lots more attention, since more people will watch the generic windows and windows-10 tags. There is of course a trade-off: you used three tags to convey the same information you could have done with one. However, it's hard to understate how many more views the generic tags will bring to your question.
Don't try to summarize your question using the tags. The point of tags on Stack Exchange is to help other interested users find your question by sorting it into clear, specific categories. This is not the same as indexing or summarizing the question. The differences are subtle, but important. Unlike many other formats like video, search engines can find your question using the text.
Don't use tags that are not about your question. Again, the point of tags here is to help sort your question into clear, specific categories. Don't just slap a tag on your question just because it happens to mention a certain topic; only use tags that are actually about your question's topic. Tags that the question is not about don't belong.
Don't use your username for a tag.
Don't use meta tags. Even if it already exists. A meta tag is any tag that doesn't categorize the content of the question, but simply indicates the nature of the question. Examples include homework or hard-question.
Don't violate any site-specific policies regarding tag usage. For example, some tags may be slated for manual community removal from all questions per procedures defined by the site community, and Super User prohibits tags that are simply the name of a company and not a specific product.
Replaces spaces with dashes (-) to combine multiple words into a single word (i.e, Tag "Unit Testing" as "unit-testing").
Avoid punctuation. This can make it difficult to use the tag in a URL
A number of tags predate the guidelines, and are therefore formatted differently. But that doesn't mean you shouldn't follow these rules when creating tags. If you find one of these, you should file a request on the per-site meta requesting that the tag be renamed to meet the guidelines.
When naming a tag, think about how someone would search for that subject. In most cases, this means typing out the full name, but you may also want to use the abbreviation in some cases. For example, "css" is probably more appropriate than "cascading-style-sheets".
Retagging a question is making any edit to a question that changes the tags.
Do not retag a question if you are not going to add value to the question information by doing so.
Do retag questions to use well-known and popular tags that are appropriate for the question.
Multi-word tags should have the words separated by a dash, unless it is a proper name that does not have a space between the words. (I.e., [sql-server] and [mysql] are correct)
Avoid creating new tags, if at all possible. Double-check your spelling if necessary. Some tags could have local variations (i.e., color or colour), some are predominantly singular or predominantly plural (i.e., color or colors), and some may have synonyms with completely different spellings (i.e., newbie or beginner). If a similar concept tag is already available, please do not create a new tag: use the existing tag instead. The goal is to categorize your question.