A quality like are that which can be crawl by the Google and other most popular search engine and that link can be counted by search engine for calculating the PR for your website so you can get the ultimate advantages for that. Traffic from that link also advantageous.
sites with relevant and related content. Sites with high pr increase your rank. The best links are the free links, who are shared by people who like your website.
Quality Links are those coming from websites that are relevant to your site. And of course websites with authority.
Quality linking means that the link you have generated for your website can be crawl by the user. Also you are getting the nice web traffic from the backlink which you have generated.
Quality back link means your site linked to high pr website and that site have good domain authority/page authority.
I have not heard before that you can get quality links from .edu and gov. sites as suggested by Olalinks. I wonder what others know about that method being good for quality. I do know that quality links can be obtained by social booking marking and good PR article directories.
Quality links are those which we get from do follow,high PR and relevant websites.That websites do not link farms.