SEM, SEO et SEA are all English acronyms. Even though they look alike, they are about different topics and it is important to distinguish them.
SEO = Search Engine Optimization. It is about optimizing your website so it appears in the natural search engines results.
SEA = Search Engine Advertising, unlike the previous definition, it is about optimizing the results of sponsored links (paid links)
SEM = Search Engine Marketing, to make it simple: SEM = SEO + SEA, it includes all the techniques that allow you to optimize your visibility on search engines.