Links from high reputation and well-established sites. High reputation would be from .edu, .gov domains etc (but don't be tempted to buy from unethical link whores selling links on sites that don't belong to them). These sites are high reputation because it's assumed the content has been checked, verified and has some credibility. Links from well-established sites on their own domain are also good - well established would be 1 year or older in my eyes. When I buy links I nearly always check the Alexa traffic rank of the site. You want this value is as low as possible, and I tend to aim for those less than 100,000.
Quality links are links from sites that are ranked well and same topic as your site. One way links are best when ever you can get them. They are text links and not redirections. You should have different text in your links to help with search engines.
Quality = High PR + Related content to your site high pr mean there are lots of links and traffic and (somehow) guarrante that the site got a good content. realted related content mean the site got the similar content or at the same categories as your site. not just like put business site link in a sport site....
i think quality link is links from high pr and related sites to your site. there are two criterias . one of them is high pr and the second is relevance