Search engine optimization or SEO refers to improving a web sites' natural and organic ranking in the search engine results page. This more affordable method is often slow and new web sites may take up to a year to achieve decent rankings in Google.
Search engine marketing or SEM, refers mainly to the text advertisements which appear at the top or on the right hand side of search engine results pages. These advertising links are purchased mainly from Google (Adwords) and Yahoo (Overture). They then in turn publish these ads on their secondary ad network as well. For example AOL displaying Adwords.
SEO refers to Search engine Optimization.This are the techniques that are applied to make the website to rank high in google.
SEM refers to Search Engine Marketing.This is the process of increasing the visibility of website through internet promotions mainly with paid advertisement.
Search engine optimization(SEO) is a type of internet marketing where the ranking of the website is improved using unpaid methods.
Search engine marketing(SEM) is a type of internet marketing where the website is promoted through search engines, using both unpaid and paid methods.
SEO or SEM
Have the completely differently role and it works on the nature and goals of your business.
SEO gives you the top ranking in the serp and provides traffic to your site without investing money.
SEM gives you the opprtunity to drive sales and other actions which will be beneficial for business by advertising you have to pay only when someone respond to your ad.
SEO - (Search Engine Optimization) - It means of optimizing your website so that you get organic (free) traffic from the search engines.
SEM - (Search Engine Marketing) - It is an online marketing strategy that involves getting traffic, paid and unpaid, from the search engines.
"SEM is a form of Internet marketing that involves the promotion of websites by increasing their visibility in search engine results pages (SERPs) through optimization and advertising."
Both strategy help to increase visibility in the search engine and gain huge genuine traffic on your website. So SEO work likes free or SEM run paid campaign to gain effective results.