Structured data21 is code that you can add to your sites' pages to describe your content to search engines, so they can better understand what's on your pages. Search engines can use this understanding to display your content in useful (and eye-catching!) ways in search results. That, in turn, can help you attract just the right kind of customers for your business.

Optimization techniques are highly tuned to the dominant search engines in the target market. The search engines' market shares vary from market to market, as does competition. In 2003, Danny Sullivan stated that Google represented about 75% of all searches.[64] In markets outside the United States, Google's share is often larger, and Google remains the dominant search engine worldwide as of 2007.[65] As of 2006, Google had an 85–90% market share in Germany.[66] While there were hundreds of SEO firms in the US at that time, there were only about five in Germany.[66] As of June 2008, the market share of Google in the UK was close to 90% according to Hitwise.[67] That market share is achieved in a number of countries.
To avoid undesirable content in the search indexes, webmasters can instruct spiders not to crawl certain files or directories through the standard robots.txt file in the root directory of the domain. Additionally, a page can be explicitly excluded from a search engine's database by using a meta tag specific to robots (usually ). When a search engine visits a site, the robots.txt located in the root directory is the first file crawled. The robots.txt file is then parsed and will instruct the robot as to which pages are not to be crawled. As a search engine crawler may keep a cached copy of this file, it may on occasion crawl pages a webmaster does not wish crawled. Pages typically prevented from being crawled include login specific pages such as shopping carts and user-specific content such as search results from internal searches. In March 2007, Google warned webmasters that they should prevent indexing of internal search results because those pages are considered search spam.[47]
Paid search advertising has not been without controversy and the issue of how search engines present advertising on their search result pages has been the target of a series of studies and reports[23][24][25] by Consumer Reports WebWatch. The Federal Trade Commission (FTC) also issued a letter[26] in 2002 about the importance of disclosure of paid advertising on search engines, in response to a complaint from Commercial Alert, a consumer advocacy group with ties to Ralph Nader.
Imagine that you've created the definitive Web site on a subject -- we'll use skydiving as an example. Your site is so new that it's not even listed on any SERPs yet, so your first step is to submit your site to search engines like Google and Yahoo. The Web pages on your skydiving site include useful information, exciting photographs and helpful links guiding visitors to other resources. Even with the best information about skydiving on the Web, your site may not crack the top page of results on major search engines. When people search for the term "skydiving," they could end up going to inferior Web sites because yours isn't in the top results.
Users will occasionally come to a page that doesn't exist on your site, either by following a broken link or typing in the wrong URL. Having a custom 404 page30 that kindly guides users back to a working page on your site can greatly improve a user's experience. Your 404 page should probably have a link back to your root page and could also provide links to popular or related content on your site. You can use Google Search Console to find the sources of URLs causing "not found" errors31.
As the number of sites on the Web increased in the mid-to-late 1990s, search engines started appearing to help people find information quickly. Search engines developed business models to finance their services, such as pay per click programs offered by Open Text[7] in 1996 and then[8] in 1998. later changed its name[9] to Overture in 2001, was purchased by Yahoo! in 2003, and now offers paid search opportunities for advertisers through Yahoo! Search Marketing. Google also began to offer advertisements on search results pages in 2000 through the Google AdWords program. By 2007, pay-per-click programs proved to be primary moneymakers[10] for search engines. In a market dominated by Google, in 2009 Yahoo! and Microsoft announced the intention to forge an alliance. The Yahoo! & Microsoft Search Alliance eventually received approval from regulators in the US and Europe in February 2010.[11]
Social networking sites such as Facebook, Instagram, Twitter, MySpace etc. have all influenced the buzz of word of mouth marketing. In 1999, Misner said that word-of mouth marketing is, "the world's most effective, yet least understood marketing strategy" (Trusov, Bucklin, & Pauwels, 2009, p. 3).[79] Through the influence of opinion leaders, the increased online "buzz" of "word-of-mouth" marketing that a product, service or companies are experiencing is due to the rise in use of social media and smartphones. Businesses and marketers have noticed that, "a persons behaviour is influenced by many small groups" (Kotler, Burton, Deans, Brown, & Armstrong, 2013, p. 189). These small groups rotate around social networking accounts that are run by influential people (opinion leaders or "thought leaders") who have followers of groups. The types of groups (followers) are called:[80] reference groups (people who know each other either face-to-face or have an indirect influence on a persons attitude or behaviour); membership groups (a person has a direct influence on a person's attitude or behaviour); and aspirational groups (groups which an individual wishes to belong to).

In some contexts, the term SEM is used exclusively to mean pay per click advertising,[2] particularly in the commercial advertising and marketing communities which have a vested interest in this narrow definition. Such usage excludes the wider search marketing community that is engaged in other forms of SEM such as search engine optimization and search retargeting.
In early 2012, Nike introduced its Make It Count social media campaign. The campaign kickoff began YouTubers Casey Neistat and Max Joseph launching a YouTube video, where they traveled 34,000 miles to visit 16 cities in 13 countries. They promoted the #makeitcount hashtag, which millions of consumers shared via Twitter and Instagram by uploading photos and sending tweets.[25] The #MakeItCount YouTube video went viral and Nike saw an 18% increase in profit in 2012, the year this product was released.
When would this be useful? If your site has a blog with public commenting turned on, links within those comments could pass your reputation to pages that you may not be comfortable vouching for. Blog comment areas on pages are highly susceptible to comment spam. Nofollowing these user-added links ensures that you're not giving your page's hard-earned reputation to a spammy site.
Marketers target influential people on social media who are recognised as being opinion leaders and opinion-formers to send messages to their target audiences and amplify the impact of their message. A social media post by an opinion leader can have a much greater impact (via the forwarding of the post or "liking" of the post) than a social media post by a regular user. Marketers have come to the understanding that "consumers are more prone to believe in other individuals" who they trust (Sepp, Liljander, & Gummerus, 2011). OL's and OF's can also send their own messages about products and services they choose (Fill, Hughes, & De Francesco, 2013, p. 216). The reason the opinion leader or formers have such a strong following base is because their opinion is valued or trusted (Clement, Proppe, & Rott, 2007). They can review products and services for their followings, which can be positive or negative towards the brand. OL's and OF's are people who have a social status and because of their personality, beliefs, values etc. have the potential to influence other people (Kotler, Burton, Deans, Brown, & Armstrong, 2013, p. 189). They usually have a large number of followers otherwise known as their reference, membership or aspirational group (Kotler, Burton, Deans, Brown, & Armstrong, 2013, p. 189. By having an OL or OF support a brands product by posting a photo, video or written recommendation on a blog, the following may be influenced and because they trust the OL/OF a high chance of the brand selling more products or creating a following base. Having an OL/OF helps spread word of mouth talk amongst reference groups and/or memberships groups e.g. family, friends, work-friends etc. (Kotler, Burton, Deans, Brown, & Armstrong, 2013, p. 189).[81][82][83][84][84][84] The adjusted communication model shows the use of using opinion leaders and opinion formers. The sender/source gives the message to many, many OL's/OF's who pass the message on along with their personal opinion, the receiver (followers/groups) form their own opinion and send their personal message to their group (friends, family etc.) (Dahlen, Lange, & Smith, 2010, p. 39).[85]

Yelp consists of a comprehensive online index of business profiles. Businesses are searchable by location, similar to Yellow Pages. The website is operational in seven different countries, including the United States and Canada. Business account holders are allowed to create, share, and edit business profiles. They may post information such as the business location, contact information, pictures, and service information. The website further allows individuals to write, post reviews about businesses, and rate them on a five-point scale. Messaging and talk features are further made available for general members of the website, serving to guide thoughts and opinions.[49]
In 2013, the Tenth Circuit Court of Appeals held in, Inc. v. 1-800 Contacts, Inc. that online contact lens seller did not commit trademark infringement when it purchased search advertisements using competitor 1-800 Contacts' federally registered 1800 CONTACTS trademark as a keyword. In August 2016, the Federal Trade Commission filed an administrative complaint against 1-800 Contacts alleging, among other things, that its trademark enforcement practices in the search engine marketing space have unreasonably restrained competition in violation of the FTC Act. 1-800 Contacts has denied all wrongdoing and is scheduled to appear before an FTC administrative law judge in April 2017.[29]
You can confer some of your site's reputation to another site when your site links to it. Sometimes users can take advantage of this by adding links to their own site in your comment sections or message boards. Or sometimes you might mention a site in a negative way and don't want to confer any of your reputation upon it. For example, imagine that you're writing a blog post on the topic of comment spamming and you want to call out a site that recently comment spammed your blog. You want to warn others of the site, so you include the link to it in your content; however, you certainly don't want to give the site some of your reputation from your link. This would be a good time to use nofollow.

Some search engines have also reached out to the SEO industry, and are frequent sponsors and guests at SEO conferences, webchats, and seminars. Major search engines provide information and guidelines to help with website optimization.[18][19] Google has a Sitemaps program to help webmasters learn if Google is having any problems indexing their website and also provides data on Google traffic to the website.[20] Bing Webmaster Tools provides a way for webmasters to submit a sitemap and web feeds, allows users to determine the "crawl rate", and track the web pages index status.