The major difference between Search Engine Marketing (SEM) and Search Engine optimization (SEO) is that SEM is a paid strategy and SEO is an organic strategy. Search Engine marketing is the strategies to attract customer's attention while Search Engine Optimization is the strategy of making more interesting the Search engines. SEM needs payment on each time while SEO needs no payment. SEM is visible to some limited customers while SEO is for everyone. SEM workd immediately whereas SEO needs some time to work.