Why Are Back Links So Important To The Success Of A Website?
For starters, having relevant, reputable sites linking back to a site is one indication of the success and popularity of that site in the eyes of the general online public. And as long as these incoming links are from a quality source then the website's page rank will improve, thus improving its location within online search results, as well.
What Defines A Quality Back Link?
There really is no true definition or standard to be used in determining whether or not a site can be considered high quality - you'll just have to use your best judgment on this one, as well as a little bit of trial and error. The more popular and well-established sites will obviously be of the highest quality, though it can sometimes be difficult getting your link posted with them.
You'll also want to consider the relevance of the websites that are providing you with back links, too. If these sites have content that is similar to yours then they are likely to be more beneficial when compared to some random, off-topic website.