SEO, search engine optimization is all about optimizing your website to get better rankings. It is the process of getting free traffic from engine engines.
SEM, search engine marketing is the goal of getting traffic either for free or through the paid medium.
The difference between SEO and SEM is that search engine optimization is part of search engine marketing. Both the processes are mainly used in increasing the visibility in search engines.
SEARCH ENGINE OPTIMIZATION
An optimized website is better understood by search engine crawlers.
The two pillars of SEO are on-page SEO and off-page SEO.
On-page SEO can be used in your website as you can make appropriate changes your website to increase traffic.
Off-page SEO refers of getting votes from other websites. It is the process of getting visibility from other websites
It is like a ranking system where the web site who has the most votes ranks higher.Votes in this reference are links coming from other web sites to your site. That is also the reason why off-site SEO is referred to as link building.
SEARCH ENGINE MARKETING
Search Engine Marketing is over and above SEO but it’s only a subset of your overall Internet Marketing Strategy.
SEM traffic is considered the most important source of Internet traffic because it is targeted. It is widely accepted that people use search engines to find a solution to a problem, an answer to their question or to learn how to do something.So when searches are clicked on a web site related to the search results or click on an ad, there are in favour to convert. In accordance of the displayed web sites and ads makes SEM traffic more valuable than any of the other sources.
Like, Facebook and twitter have been trying to optimize their advertising platforms to utilize intended traffic but still traffic coming directly from search engines has a better ROI.
Therefore, SEO is about optimizing your website to get better rankings in the search results while SEM goes beyond SEO.