Quality Links?

Quality links are links that come from high authority sites that are relevant to your business. The site linking to your web pages should perform well on Google and have valuable content. Older websites with high traffic and high Page Rank are perceived as more authoritative.

Agreed.

Also where do you find authority sites relevant to your niche where you can place your link for free? Besides blog commenting.

After doing some research on high PR domains that are about to expire most quality links with a high pagerank seem to be from:

• companies that have a page with all their dealers
• the more serious review sites
• lists of certain branches, for example a travel agency that links out to hotels that they do business with
• sites that link out to artists / musicians
• sites that link out to software developers / apps etc
• donation pages
• paid directories, think of Yahoo, but there is also Dmoz which is free I think, not sure

This are real links that are kind of natural in Google's eyes, and obvious these links are not easy to achieve, mostly you need a real business that is connected with certain organizations to get listed.

The easier to achieve high quality links, or better said high PR links are:

• extracting juice from free web2.0 properties by using techniques such as reblogging, following other blogs (tumblr trick is a famous one), the same counts for certain social sites.
• blogcomments at high PR pages with low obl
• forumposts in threads with a high PR
• guestposts (although they mostly end up on PR0 pages they can gain PR after some time)
• pressreleases, you get listed at pr n/a page in first place at PRweb.com and news.yahoo.com, but cause other news sites pick it up they link back to PRweb and Yahoo news so these pages do gain PR and authority so you can consider it as a quality link
• high PR domains, you can buy them at Godaddy or at other places and then you can build your own site on it and link back to your own site, easy way to achieve high PR links. Btw I see many expired domains with Yahoo listings, but don't get crazy on that as Yahoo charges a yearly fee so these links will expire.
• rented homepage/blogroll links

Guess that's about it, or at least what crosses my mind right now. As you see, quality links cost money, require connections or take quiet some time.

Obvious the most easy ones (reblogging or commenting) are the less valuable ones.

nik0 has already pretty much put it as it is, but I will try and give my two cents. Quality is something very relative, and in this forum (like everywhere else) different people may have different meanings for the same word, and this is specially true for the words 'quality link'. Google doesn't usually tell us much about the way they rank websites, but in this particular case they do, so instead of taking my word for it, or anyone else's, go to the source and check it for yourself:

googlewebmastercentral.blogspot.com.es/2008/10/good-times-with-inbound-links.html

So, for Google a quality link is an editorial-given link from a content-related website. The better the content-related website is (higher PR, trusted, popular...) the more weight that link has. The key here is the word EDITORIAL, meaning there is no way you can get that link yourself, the webmaster of the other site has to decide by himself to give you the vote, I mean, the link. Plain, simple and easy. As G says, write great content and let the internet love you.

So what happened for such a simple concept to get so messed up? Well, the problem comes when you don't have great content. A lot of people (including myself) have many websites that nobody is willing to link to. Crappy content, bad design or just plain spam. And when you have that kind of websites you need to build the links yourself, and then the quality concept just plays in another different league, the league of NON-EDITORIAL links, where the higher quality link is worth very litle compared to one regular editorial link.

And in this league of built up links is where the points written by nik0 make some sense. See how he said "high PR links" instead of "high quality links". He knows what he's talking about.