A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.

designed to solicit opinions or best-practices on a particular topic, with the goal of reaching community consensus.
You need help with the use of one or more of the site's features.
260 questions
Indicates that the report has been resolved through the implementation of a feature or the fixing of a bug.
154 questions
For questions about the behavior or usage of tags, which are keywords or labels that categorize and group your question with other, similar questions.
when examining what is on-topic or off-topic on this site, and why.
113 questions
Indicates you've found an erroneous or unexpected behavior in the system that needs to be fixed.
Questions about the design of the site, whether it is a bug, feature-request or otherwise.
51 questions
You have an idea for a new feature, or for a change to the existing functionality.
47 questions
used to query the community on the specifics of a particular question asked on the main site or the meta site.
for requests to replace one tag with another existing tag. It also applies to requests that one tag should be merged into another, or simply abandoned.
42 questions
For coordinating projects to help clean up the main site.
34 questions
intentional and not subject to change
33 questions
For questions regarding the nature of a "question" on Web Applications Stack Exchange; e.g. how to ask, whether specific questions are appropriate, etc.
31 questions
for questions about tag synonyms (how they work, who can create them, etc). Use [synonym-request] tag for tag synonym requests.
29 questions
the process of migrating off-topic questions from a Stack Exchange site to another.
24 questions
for general questions about questions that have been closed because they are off-topic or otherwise do not meet the criteria defined in the FAQ.
For questions about the /review path where users can view and act on posts by other users that the system thinks may need attention.
19 questions
held to choose moderators for Stack Overflow and non-Beta Stack Exchange sites.
Proposals for new entries for the FAQ repository on this site should use this tag.
17 questions
a measurement of a user's contributions to the site, and the extent to which the community "trusts" that user with extra privileges.
16 questions
For questions about the process of editing, how edits work, and other general inquiries about the edit system.
15 questions
the process by which users can vote to close a question. You gain the ability to cast close votes when you reach 3,000 Reputation.
14 questions
for instructions and guidance on closing or migrating questions, and the reasons for those closures.
14 questions
For questions about the flagging process, how flags are handled, etc.
14 questions
Questions about dealing with "meta tags", tags that don't categorize the question, and are undesirable.
14 questions
A request to add one tag as the synonym of another tag. Please be specific about which tag should be a synonym of which. Bidirectional synonyms and cycles are implicitly disallowed, so suggest wisely.
12 questions
Users without full edit privileges can suggest edits to posts and tag wikis.
12 questions
Questions flagged as duplicate, and/or reasons to do so.
12 questions
For questions about Stack Exchange, the parent of around 150 web sites (of which Web Applications is one) in connection with Web Applications. Other questions about "the SE family" or its parent may b…
10 questions
for questions about answers: how to answer, whether specific answers are appropriate, and so forth.
10 questions
Questions about Web Applications Meta.
10 questions
small addendum to each question or answer, intentionally limited in size and formatting, and are meant to be temporary.
10 questions
Refers to user accounts either generically, or across the network.
10 questions
for questions about suspected user spam, the spam flag or combating spam as a community.
10 questions
awarded to encourage and incentivize positive community actions within the site.
9 questions