SEA (Search Engine Advertising)
SEA is a method of placing online advertisements, against paiment, on web pages that shows results from search engine queries.
There are three objectives of SEA: try to generate as many views as possible, obtain as many clics as possible and realize conversions (e.g. contact, sales ...)
SEM (Search Engine Marketing)
SEM is a form of Internet marketing that seeks to promote websites by increasing their position in search engine result pages. This is possible through the use of search engine optimization (SEO) (optimization of website layout and content) and paid advertisement (SEA)to obtain better results in search engines.
SEO (Search Engine Optimization)
SEO is used to obtain a better ranking in search engine results through pertinent keywords for your website.