SEO :
Search engine optimization is a methodology of strategies, techniques and tactics used to increase the amount of visitors to a website by obtaining a high-ranking placement in the search results page of a search engine (SERP) -- including Google, Bing, Yahoo and other search engines.
SEO helps to ensure that a site is accessible to a search engine and improves the chances that the site will be found by the search engine.
SEM :
Search engine marketing (SEM) is a form of Internet marketing that involves the promotion of websites by increasing their visibility in search engine results pages (SERPs) through optimization and advertising.SEM may use search engine optimization (SEO), that adjusts or rewrites website content to achieve a higher ranking in search engine results pages or use pay per click listings.
I prefer SEO . I use Search engine visibility servcie to increase my website rank and traffic . BTW, i got it from the provider
Tucktail.com