A sarcasm tag has its roots in html. What we're talking about is actually an end sarcasm tag. Typically, we use /sarcasm, /sarc or /s at the end of a sarcastic remark, to make it clear to others that we do not agree with the statement. You can use the full html, but it's more difficult. </sarcasm> (It won't work unless you use the proper html code for the left and right arrows).
example: Indians, blacks or anyone other than white are incapable of being racist. </sarcasm>
Without that sarcasm tag, it means that you agree with the statement.