Content posted as an answer that doesn't actually answer the question at hand may be flagged "Not an Answer". Note *bad* answers are not eligible for this kind flag.
Post that doesn't actually answer the question at hand may be flagged "Not an Answer". It may be a followup question, a comment, or additional details from the asker posted as an answer. Bad answers are still answers and should be dealt with through voting and commenting rather than flagging.
"Not an Answer" flag description, as displayed to Stack Overflow users:
This was posted as an answer, but it does not attempt to answer the question. It should possibly be an edit, a comment, another question, or deleted altogether.
Detailed explanation for the intended use of "Not an Answer" flag, as given by Joel Spolsky in Proposed improvement to flagging, for answers:
Should have been a comment - this is not an answer, it is a comment on the question or on another answer.
Should have been an edit - this is not an answer, it is additional information about the question from the original asker.
Me-too! non-answer - this answer consists of nothing more than "Me too!" or "Did anyone find the answer?"
Conversational and irrelevant - this is not an answer, just a random conversation or rant, or meaningless typing that should be deleted.
September 2012 announcement about adjustment made to flag description:
Should the not-an-answer flag description be clarified?