Quality Content: In the world of content marketing, quality content simply refers to any content that is worthy of being shared (or "linkworthy"). As this is a somewhat loaded term, the definition of quality can vary across platforms and according to users.
Quality is sometimes gauged by popularity. A link that gets shared thousands of times on Facebook may lend better content to the web than a link with very few shares... but not necessarily!
Quality is also measured by other authorities, like Google. Google's rank is determined partly by its Panda algorithm, which looks at -- among other things -- the quality of a website's design and content. Through this algorithm, sites with lower amounts of content, or "thin" content, are ranked far lower than more informative sites. While it's not always possible for webmasters to know exactly how their pages will rank according to Google's measurements, they can have a good predictor of their content's quality based on Google's Webmaster Guidelines (in addition to using good SEO practices).
Another important factor in determining quality has to do with how unique the content is. For instance, a website may include a useful guide to gaining ROI from SEO practices. If another webmaster goes to that site, copies down the information, and then rewrites it in his or her own words, that doesn't necessarily mean that the content was unique. Therefore, it may not be considered quality content because it doesn't give internet users anything that they can't find elsewhere.
To most internet analysts, however, quality content isn't enough for a business on the web. Promotion and link development, among other SEO practices, are still necessary in order to get that content noticed.