The links from such websites that have Similar Contents High Google Page Rank Less advertisement Good Traffic from Search Engines and many more ....... You must searh with this keyword on google " Quality BackLinks"
Link that are from established quality sites which are relevant to your site. For instance if your site is about cars try to get links from car related sites.
almost any backlink is a good backlink. A 'real' site acquires backlinks from all over the shop, from PR zilch to PR weyhey.
Some time PR doesnt make differance but if link comming from same kind of content or realted content with high traffic or reach keywords would be benificial and specially link is visible and all the high traffic home page link alway help improve your keyword ranking.
Concentrate on article marketing, blog commenting , social bookmarking and forum posting, these backlinks alone will do your site great credit.
Welcome to this community and glad to know that you want to share knowledge with us .well as far as my openion for quality back link must be unique and get relavancy.all content must be simple, impressive and well targeted.