Search engines use complex mathematical algorithms to interpret which websites a user seeks. In this diagram, if each bubble represents a website, programs sometimes called spiders examine which sites link to which other sites, with arrows representing these links. Websites getting more inbound links, or stronger links, are presumed to be more important and what the user is searching for. In this example, since website B is the recipient of numerous inbound links, it ranks more highly in a web search. And the links "carry through", such that website C, even though it only has one inbound link, has an inbound link from a highly popular site (B) while site E does not. Note: Percentages are rounded.

Facebook Ads and other social media ad platforms, for example, are pay-per-click platforms that do not fall under the SEM category. Instead of showing your ads to people who are searching for similar content like search ads do, social media sites introduce your product to people who happen to be just browsing through their feeds. These are two very, very different types of online advertising.
More than three billion people in the world are active on the Internet. Over the years, the Internet has continually gained more and more users, jumping from 738 million in 2000 all the way to 3.2 billion in 2015.[9] Roughly 81% of the current population in the United States has some type of social media profile that they engage with frequently.[10] Mobile phone usage is beneficial for social media marketing because of their web browsing capabilities which allow individuals immediate access to social networking sites. Mobile phones have altered the path-to-purchase process by allowing consumers to easily obtain pricing and product information in real time[11]. They have also allowed companies to constantly remind and update their followers. Many companies are now putting QR (Quick Response) codes along with products for individuals to access the company website or online services with their smart phones. Retailers use QR codes to facilitate consumer interaction with brands by linking the code to brand websites, promotions, product information, and any other mobile-enabled content. In addition, Real-time bidding use in the mobile advertising industry is high and rising due to its value for on-the-go web browsing. In 2012, Nexage, a provider of real time bidding in mobile advertising reported a 37% increase in revenue each month. Adfonic, another mobile advertisement publishing platform, reported an increase of 22 billion ad requests that same year.[12]

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.
Webmasters and content providers began optimizing websites for search engines in the mid-1990s, as the first search engines were cataloging the early Web. Initially, all webmasters only needed to submit the address of a page, or URL, to the various engines which would send a "spider" to "crawl" that page, extract links to other pages from it, and return information found on the page to be indexed.[5] The process involves a search engine spider downloading a page and storing it on the search engine's own server. A second program, known as an indexer, extracts information about the page, such as the words it contains, where they are located, and any weight for specific words, as well as all links the page contains. All of this information is then placed into a scheduler for crawling at a later date.
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