SEM

Search engine marketing, (SEM), is a form of Internet marketing that seeks to promote websites by increasing their visibility in search engine result pages  (SERPs) through the use of, paid placement, contextual advertising, and paid inclusion.

Search engine optimization (SEO) is “optimizing” website pages to achieve higher ranking in search results via the process of selecting specific keyword expressions associated to the website ^[3] SEM is utilizing various means of “marketing” a website in order for it to become more relevant in regard to search engine searches and their rankings. It should be asserted that SEM is NOT SEO and vice versa.

source Wikipedia

SEO

There’s been a lot of shouting about Search Engine Optimisation (SEO) in recent times and indeed, SEO is one of the keys to maximising the effectiveness of your web presence. Good SEO will have your website appearing at the top of search engine rankings for the keywords that matter to your business.

Simply having a great website isn’t enough any more. Many websites are sitting on gold mines, with excellent content and usability but the crucial missing link is that they haven’t been structured in the way search engines like. As a result, they languish in obscurity. With the vast majority of web users getting to their destination via search engines, SEO should be a key component of your online strategy.