Where the free Google tools can provide complementary value is in fact-checking. If you're checking out more than one of these SEO tools, you'll quickly realize this isn't an exact science. If you were to look at the PA, DA, and keyword difficulty scores across KWFinder.com, Moz, SpyFu, SEMrush, Ahrefs, AWR Cloud, and Searchmetrics for the same set of keywords, you might get different numbers across each metric separated by anywhere from a few points to dozens. If your business is unsure about an optimization campaign on a particular keyword, you can cross-check with data straight from a free AdWords account and Search Console. Another trick: Enable Incognito mode in your browser along with an extension like the free Moz Toolbar and you can run case-by-case searches on specific keywords to get an organic look at your target search results page.

Mike Levin is the Senior SEO Director at Ziff Davis, PCMag's parent company. His career goes back 25 years to the halls of Commodore Computers, as an original Amiga fanboy to be beamed up by the mothership just as it imploded. Over his past 10 years in NYC, Mike's highlights have included leading the Apple Store, Kraft and JCPenney SEO accounts whi... See Full Bio

Many blogging software packages automatically nofollow user comments, but those that don't can most likely be manually edited to do this. This advice also goes for other areas of your site that may involve user-generated content, such as guest books, forums, shout-boards, referrer listings, etc. If you're willing to vouch for links added by third parties (for example, if a commenter is trusted on your site), then there's no need to use nofollow on links; however, linking to sites that Google considers spammy can affect the reputation of your own site. The Webmaster Help Center has more tips on avoiding comment spam40, for example by using CAPTCHAs and turning on comment moderation.

Search engine optimization is a method for sustainably influences search engine rankings. Google and other search engines calculate their search results for keywords using highly complex algorithms. The individual ranking factors and their weighting within the ranking calculation are well-guarded intellectual property that belongs to the search engines and is not publicly disclosed.

The terms SEO experts often start with are page authority (PA) and domain authority (DA). DA, a concept in fact coined by Moz, is a 100-point scale that predicts how well a website will rank on search engines. PA is the modern umbrella term for what started as Google's original PageRank algorithm, developed by co-founders Larry Page and Sergey Brin. Google still uses PageRank internally but has gradually stopped supporting the increasingly irrelevant metric, which it now rarely updates. PA is the custom metric each SEO vendor now calculates independently to gauge and rate (again, on a scale of 100) the link structure and authoritative strength of an individual page on any given domain. There is an SEO industry debate as to the validity of PA and DA, and how much influence the PageRank algorithm still holds in Google results (more on that in a bit), but outside of Google's own analytics, they're the most widely accepted metrics out there.

A website or URL’s ranking for keywords or keyword combinations varies from search engine to search engine. A domain may rank for a certain keyword in the top 3 on Bing, but not even be on the first page of the Google search results for the same keyword. Of course, the same is true of all search engines – Bing, Google, Yahoo and every other search engine uses its own method for calculating rankings and therefore ranks websites differently.

This is also where you can see Google's ML algorithms at work. Powered by Google Cloud Platform, the way Quick Answers and Featured Snippets are extracted gets increasingly smarter as Google introduces new innovations in deep learning and neural networks. These constantly evolving algorithms are baked into how the search engine surfaces information.

Ranking refers to the process search engines use to determine where a particular piece of content should appear on a SERP. Search visibility refers to how prominently a piece of content is displayed in search engine results. Highly visible content (usually the content that ranks highest) may appear right at the top of organic search results or even in a featured snippet, while less-visible content may not appear until searchers click to page two and beyond