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.
A tag for questions that may not necessarily have a clear-cut right or wrong answer and are often subjective. If it's not a bug, feature-request or request for assistance, it is probably a discussion.
keywords or labels that categorize and group your question with other, similar questions.
for proposals of new features on the site, or requests for a change to an existing feature.
for questions about, well, questions: how to ask, whether specific questions are appropriate, and so forth.
for questions about answers: how to answer, whether specific answers are appropriate, and so forth.
"Design" refers to front-end elements, such as page layout, coloring conventions, etc. For questions about database structure, algorithms in the code, please use the "architecture" tag instead.
for questions about suspected user spam, the spam flag or combating spam as a community.
small addendum to each question or answer, intentionally limited in size and formatting. Comments are second-class citizens.
the process of bringing a post to a community moderator's attention for any reason.
a way to get additional attention for a question by offering some of your own reputation for great answers.
a special celebration that typically runs from December 15th until January 4th each year.
Users with sufficient reputation can flag posts that have quality problems which they are unable to address themselves, thereby bringing them to the attention of the site's moderators.
held to choose moderators for Stack Overflow and non-Beta Stack Exchange sites.
Users without full edit privileges can suggest edits to posts and tag wikis.
the process of migrating off-topic questions from a Stack Exchange site to another.
available to users with a reputation of 10000.
used for questions about meta.drupal.stackexchange.com.