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.

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a globally accessible avatar storage service, used, in particular, by StackOverflow.
used to break a long string of text into two lines of text. In html, the <br> tag is used for a line-break.
A DNS-based Blackhole List (DNSBL) or Real-time Blackhole List (RBL) is an effort to stop email spamming. It is a "blacklist" of locations on the Internet reputed to send email spam.