A postscript (from post scriptum, a Latin expression meaning "after writing" and abbreviated P.S. or p.s.) is a sentence, paragraph, or occasionally many paragraphs added, often hastily and incidentally, after the signature of a letter or (sometimes) the main body of an essay or book. In a book or essay, a more carefully-composed addition (e.g., for a second edition) is called an afterword. An afterword, not usually called a postscript, is written in response to critical remarks on the first edition. The word has, poetically, been used to refer to any sort of addendum to some main work, even if not attached to a main work, as in S├Şren Kierkegaard's book titled Concluding Unscientific Postscript.
In the age of e-mail, postscripts have become unnecessary: any modifications or additions to the body of a letter may simply be inserted within the e-mail before sending, though the convenience of a post-scripted addition is always available. Postscripts in e-mails are most often
Source: http://en.wikipedia.org/wiki/Post Scriptum