Key Difference: SEO is a subset of SEM, where both are used to promote a website in order to draw in more visitors. Search Engine Marketing is a method used to promote websites and products online through paid advertising techniques. SEO on the other hand is the process to improve the visibility of a website in a search engine's unpaid section.

Search Engine Optimization (SEO) and Search Engine Marketing (SEM) are two words that are most commonly heard in the digital world. SEM and SEO have become prominent words for businesses in today’s technology advanced world. SEO is a subset of SEM, where both are used to promote a website in order to draw in more visitors.

 Search Engine Marketing is a method used to promote websites and products online through advertising techniques such as paid placements, contextual advertising, paid inclusions and SEO. Search Engine Optimization (SEO) is a major part of SEM. Webopedia defines ‘SEM’ as, “acts associated with researching, submitting and positioning a Web site within search engines to achieve maximum exposure of your Web site.”

SEO on the other hand is the process to improve the visibility of a website in a search engine's unpaid section, including altering the website to make it more viewer and search engine friendly. A general rule of web pages is the higher up on a search list a company’s website appears, the more visitors it receives. SEO is not only limited to text, but also includes image search, video search, academic search, local search, and news search. Webopedia defines ‘SEO’ as, “the process of increasing the amount of visitors to a website by obtaining a high-ranking placement in the search results page of a search engine (SERP). The higher a website ranks in the results of a search, the greater the chance that that site will be visited by a user.” In order to rank higher using SEO, a developer would have to use the proper keywords, and increase the amount of backlinks, other websites that have linked your page, in order to make it easier for the ‘spiders’ to pick up your website and properly index it.

Search engines use robots, spiders or crawler software in order to find different websites and add them to the database. A main difference between SEO and SEM is that SEM requires the developer of the website to pay money for placing their websites in top searches, while SEO requires a person to put the amount of time and energy, putting the proper keywords for the search engine to find the website and is free. SEO is also a slower process compared to SEM, but more beneficial; while SEM can get visitors to your websites, it may not always be relevant to what they may be searching for; whereas since SEO uses keywords that a person searches, it results in a more relevant search.

In a nutshell, SEO considers how search engines work, what a user is looking for, popularity of certain keywords typed into a search engine, and popular search engines themselves, Google currently leads as the most popular search engine. SEM adds your website to its database instantly and constantly updates it, the website is also placed in the top of the search results for viewers; though a drawback to SEM is that it may or may not be relevant to what a visitor is searching for. The largest SEM vendors are Google AdWords and The Yahoo! & Microsoft Search Alliance. Though, SEM is automatically associated with paid advertising, it also includes SEO as well as Social Media Marketing, where social media websites are used for the purposes of marketing websites and products.

An enterprise or a web page developer may use both or either in order to promote their websites, depending on a budget. While SEM is faster, it also offers short term results compared to SEO, which provides long term results. With vast resources at hand, SEM is a good option to use, but with limited resources SEO is a better option.

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