3 Answers

Positive feedback is when you are given praise and told of the good points of both yourself and your ability and how well you are doing,  negative feedback is basically the oposite you get informed of the bad points or points that you need to improve on.

"Feedback" is a response from others, an evaluation of sorts. One purpose of feedback could be to help improve a product or plan.  It can sometimes, in the right setting, be called advice. Feedback is usually requested; offering up feedback when it's not requested is often interpreted as being critical or negative. .Sometimes, unsolicited advice or opinion is positive. Pythonlover has given a good explanation of both. 

For buyers to leave feedback about an item is really a courtesy. Positive feedback increases the seller's feedback score, and some buyers prefer to shop from sellers with higher scores. Buyers may want to consider themselves as part of the overall shopping community. Leaving feedback -- whether positive or negative -- may not affect themselves if they aren't going to shop from the seller again, but their scoring and notes will be read by other potential shoppers considering a purchase.