QUOTE: “The score is determined from quantities indicating user actions of seeking out and preferring particular sites and the resources found in particular sites. *****A site quality score for a particular site**** can be determined by computing a ratio of a numerator that represents user interest in the site as reflected in user queries directed to the site and a denominator that represents user interest in the resources found in the site as responses to queries of all kinds The site quality score for a site can be used as a signal to rank resources, or to rank search results that identify resources, that are found in one site relative to resources found in another site.” Navneet Panda, Google Patent

******” Quote from Google: One other specific piece of guidance we’ve offered is that low-quality content on some parts of a website can impact the whole site’s rankings, and thus removing low-quality pages, merging or improving the content of individual shallow pages into more useful pages, or moving low-quality pages to a different domain could eventually help the rankings of your higher-quality content. GOOGLE ******

NOTE, in 2019, the HTML title element you choose for your page, may not be what Google chooses to include in your SERP snippet. The search snippet title and description is very much QUERY & DEVICE dependant these days. Google often chooses what it thinks is the most relevant title for your search snippet, and it can use information from your page, or in links to that page, to create a very different SERP snippet title.
Since heading tags typically make text contained in them larger than normal text on the page, this is a visual cue to users that this text is important and could help them understand something about the type of content underneath the heading text. Multiple heading sizes used in order create a hierarchical structure for your content, making it easier for users to navigate through your document.
Thick & Unique Content – There is no magic number in terms of word count, and if you have a few pages of content on your site with a handful to a couple hundred words you won’t be falling out of Google’s good graces, but in general recent Panda updates in particular favor longer, unique content. If you have a large number (think thousands) of extremely short (50-200 words of content) pages or lots of duplicated content where nothing changes but the page’s title tag and say a line of text, that could get you in trouble. Look at the entirety of your site: are a large percentage of your pages thin, duplicated and low value? If so, try to identify a way to “thicken” those pages, or check your analytics to see how much traffic they’re getting, and simply exclude them (using a noindex meta tag) from search results to keep from having it appear to Google that you’re trying to flood their index with lots of low value pages in an attempt to have them rank.
Moreover: if you don’t have to, don’t change your URLs. Even if your URLs aren’t “pretty,” if you don’t feel as though they’re negatively impacting users and your business in general, don’t change them to be more keyword focused for “better SEO.” If you do have to change your URL structure, make sure to use the proper (301 permanent) type of redirect. This is a common mistake businesses make when they redesign their websites.
QUOTE: “Supplementary Content contributes to a good user experience on the page, but does not directly help the page achieve its purpose. SC is created by Webmasters and is an important part of the user experience. One common type of SC is navigation links which allow users to visit other parts of the website. Note that in some cases, content behind tabs may be considered part of the SC of the page.” Google Search Quality Evaluator Guidelines 2017

QUOTE:  “Tell visitors clearly that the page they’re looking for can’t be found. Use language that is friendly and inviting. Make sure your 404 page uses the same look and feel (including navigation) as the rest of your site. Consider adding links to your most popular articles or posts, as well as a link to your site’s home page. Think about providing a way for users to report a broken link. No matter how beautiful and useful your custom 404 page, you probably don’t want it to appear in Google search results. In order to prevent 404 pages from being indexed by Google and other search engines, make sure that your webserver returns an actual 404 HTTP status code when a missing page is requested.” Google, 2018
SEO techniques can be classified into two broad categories: techniques that search engine companies recommend as part of good design ("white hat"), and those techniques of which search engines do not approve ("black hat"). The search engines attempt to minimize the effect of the latter, among them spamdexing. Industry commentators have classified these methods, and the practitioners who employ them, as either white hat SEO, or black hat SEO.[50] White hats tend to produce results that last a long time, whereas black hats anticipate that their sites may eventually be banned either temporarily or permanently once the search engines discover what they are doing.[51]
SEM is a broader term than SEO, and is used to encompass different options available to use a search engine’s technology, including paid ads. SEM is often used to describe acts associated with researching, submitting and positioning a website within search engines.  It includes things such as search engine optimization, paid listings and other search-engine related services and functions that will increase exposure and traffic to your Web site. 
Page and Brin founded Google in 1998.[23] Google attracted a loyal following among the growing number of Internet users, who liked its simple design.[24] Off-page factors (such as PageRank and hyperlink analysis) were considered as well as on-page factors (such as keyword frequency, meta tags, headings, links and site structure) to enable Google to avoid the kind of manipulation seen in search engines that only considered on-page factors for their rankings. Although PageRank was more difficult to game, webmasters had already developed link building tools and schemes to influence the Inktomi search engine, and these methods proved similarly applicable to gaming PageRank. Many sites focused on exchanging, buying, and selling links, often on a massive scale. Some of these schemes, or link farms, involved the creation of thousands of sites for the sole purpose of link spamming.[25]

It helps to improve your ranking for certain keywords. If we want this article to rank for the term ’SEO basics’ then we can begin linking to it from other posts using variations of similar anchor text. This tells Google that this post is relevant to people searching for ‘SEO basics’. Some experts recommend varying your anchor text pointing to the same page as Google may see multiple identical uses as ‘suspicious’.
Mike Murray has shaped online marketing strategies for hundreds of businesses since 1997, including Fortune 500 companies. A former journalist, he has led SEO studies and spoken at regional and national Internet conferences. Founder of Online Marketing Coach, Mike is passionate about helping clients identify their best opportunities for online marketing success based on their strengths, his advice and industry trends. You can find him at his blog, Online Marketing Matters or on Twitter @mikeonlinecoach.
QUOTE: “The duration performance scores can be used in scoring resources and websites for search operations. The search operations may include scoring resources for search results, prioritizing the indexing of websites, suggesting resources or websites, protecting particular resources or websites from demotions, precluding particular resources or websites from promotions, or other appropriate search operations.” A Panda Patent on Website and Category Visit Durations
SEO is also about making your search engine result relevant to the user's search query so more people click the result when it is shown in search. In this process, snippets of text and meta data are optimized to ensure your snippet of information is appealing in the context of the search query to obtain a high CTR (click through rate) from search results.
QUOTE: “(Google Panda) measures the quality of a site pretty much by looking at the vast majority of the pages at least. But essentially allows us to take quality of the whole site into account when ranking pages from that particular site and adjust the ranking accordingly for the pages. So essentially, if you want a blunt answer, it will not devalue, it will actually demote. Basically, we figured that site is trying to game our systems, and unfortunately, successfully. So we will adjust the rank. We will push the site back just to make sure that it’s not working anymore.”  Gary Illyes – Search Engine Land
Since heading tags typically make text contained in them larger than normal text on the page, this is a visual cue to users that this text is important and could help them understand something about the type of content underneath the heading text. Multiple heading sizes used in order create a hierarchical structure for your content, making it easier for users to navigate through your document.