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 free (open source) automated testing suite for web applications across different browsers and platforms.
for browsing questions that talk about whether things are on or off-topic. It's about site-boundaries. Mainly the purpose is for refining searches.
Adding tags to a new or existing question
for questions regarding off topic questions, including whether a question is on topic and what to do with off topic questions.