People often get confused between the terms Seo and Sem and take both of them to be the same but actually there are differences between these two terms.
Seo stands for Search Engine Optimization
Seo gives website organic rankings on search engines
Seo helps to understand the ranking algorithm working for the search engines
Seo does not uses Adwords or other paid methods of advertisement on search engines
Seo results take time to achieve, normally between 3-6 months or more
Seo makes a website popular on the entire World Wide Web by creating references and backlinks
Even if you stop doing Seo, your site may keep its ranking for some days or months depending upon the competition and algorithmic updates
Sem stands for Search Engine Marketing
Sem gives a website inorganic rankings on search engines
Sem does not feels the need to do so
Sem uses Adwords and other paid methods of advertisement on search engines
Sem can your site ranked soon as it opts for paid advertising.
Sem do not uses the techniques of creating references or backlinks
The moment you stop Sem, your site’s position in Serps are gone. (Leaving the exception that your site is popular and already ranks on search engines)