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.
Yeah, I DIDN'T do you mom last night.
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.
Person A: You're so smart.
Person B: Thanks.
Person A: Whoops, forgot to include my sarcasm tags. <sarcasm> You're so smart. </sarcasm>