In SEO,we use some unpaid methods to generate traffic or improve ranking.Whereas in SEM we use both paid as well as unpaid techniques to get results.

Same question all over the internet.....

SEO is search engine optimization and SEM is search engine marketing. SEO is effective part of search engine marketing and comes in free way of doing search engine marketing whereas SEM have other paid ways of doing marketing like PPC, and others. So SEM have two techniques of doing search engine marketing, First is paid and second is unpaid. SEO comes in un-paid way of doing marketing. So no difference...

If we refer mathmatics then SEO is a subset of SEM. Search Engine marketing includes every method of promoting a website like PPC, SEO, banner ads and SMO.

Basically, SEM is paid marketing process in search engine, SEM encompasses SEO, paid advertising, Social Media Marketing, Blog marketing and Web analytics. Where SEO is a long term strategy that includes using both on and off page factors such as strategic linking, content, addressing site structure / navigation, and technical search friendliness issues.