Why ? xx or other

Hi I am new here I just want to know why in certain url. address given by members of the forum is odd,that is why instead of giving the full www or http they put a 'x' on either the 'w' or 't'. I do not understand why it is so difficult to correct it.is there a reason for this ? I sometime overlook it and paste to the address bar and the page could not found and looking closer I find the address is wrong. Thanks.