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.
Unanswered questions are those questions which have no up-voted or accepted answers.
Community Wiki is a flag that can be applied to answers (and questions) which lowers the barrier for editing and prevents any rep gain/loss from voting.
Briefly, a hackathon brings people together to use technology to transform ideas into reality.