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.
For questions about the Winter Bash (or holiday hats) to celebrate the end of the year.
"The Dupehammer" refers to the ability of tag gold badge owners to unilaterally close and reopen duplicates which were asked in tags of which they have a golden badge.
Questions related to compatibility differences across different operating systems (OSX, Windows, Linux, etc.)