I noticed that visiting 'Unanswered' questions shows even questions with answers. Does an answer have to be checked in order to be counted as an answer? Does it matter of the only answer has a negative vote count?
Unanswered questions are those questions that have not been accepted by the person who proposed that particular question. Sometimes an OP simply is waiting to see if a better answer will appear, while sometimes the OP never returns to the site.
An accepted answer is not necessarily the best answer. It simply means that for the OP that that particular response is best suited for his desired information.
An unanswered question is one that has no upvoted answers. (I'm not sure if the asker accepting an answer also affects this, if that answer isn't upvoted.) Questions that have 0-score (or lower-scoring) answers are treated as unanswered so that people who come along later -- like you -- will take a look and perhaps be able to offer an answer.