Difference between SEO and SEM

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Difference between SEO and SEM
SEO is aimed at continuously refining the website in order to get friendly with both the search engine indexing algorithms as well as the user, thereby driving traffic to the site.
SEM on the other hand is aimed purely at marketing and advertising the business represented by the website, and has nothing to do with the rankings or the contents of the website.
SEO is purely organic in nature in the sense the results are slow, consistent and quite hard to achieve and maintain.
SEM mostly comprises paid marketing services to get the required impact.
Tons of relevant, unique and keyword-rich content with appropriate titles, tags, links and images is the backbone of SEO.
SEM is hardly worried about the content. Lesser the content, just a few fields to capture the details for a subscription or registration and the job is done.