Used in chats and conversations every day, "</sarcasm>" is used to denote sarcasm. In HTML programming, tages are ended with a "/", such as </b> for bold text. Usually one would say a sarcastic remark, and then use </sarcasm> to indicate that he has finished his sarcasm. It pwns your mom.

Used after a sarcastic retort to inform those too stupid to detect sarcasm by themselves. This is never spoken, only written, and generally only used in online forums or instant messenging and such.