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.
those questions which have no up-voted or accepted answers.
Briefly, a hackathon brings people together to use technology to transform ideas into reality.
a flag that can be applied to answers (and questions) which lowers the barrier for editing and prevents any rep gain/loss from voting.
a reference to data or content that the reader can directly follow either by clicking, hovering over, or that is followed automatically. (Ref. Wikipedia)
OpenID - the Internet Identity Layer