What is the difference between SEO and SEM?

[TomClarke]

One big difference  between SEO and SEM is cost effectiveness in the long run. You can get short term benefits through SEM in the long run, you cannot make do without SEO. It also btings quality traffic to your website than SEM which often brings visitors that are casual are not interested in your product.
While SEM is costly, SEO is for free but time consuming. You are rewarded for your efforts in the long run when you use SEO techniques. Without SEM however, no site will be viewed and would make all SEO attempts futile. Thus it would be fair to say that both SEO and SEM are necessary to make a site search engine friendly and to generate more traffic for your site.

[LaurentHector]

SEO: stands for Search Engine Optimization - Optimizing search engines by optimizing Onpage and offpage aims to bring your website to the highest ranking on the region's natural results search engines like Google, yahoo , Bing. The goal of SEO is to bring the site to the top of search results according to some keyword (keywword) specific. For example, a company that provides SEO services want the keyword "SEO services, cheap seo services, seo gia re servicers" TOP 10 google.com.vn LEP, they need to do SEO for TOP with words that key.
 
+ SEM: SEM = SEO + ADWORDS + LBS + VSM. Such is the broader concept SEM and SEO included therein. In other words, SEO is a branch of SEM. SEM is the synthesis of a variety of methods aimed at helping marketing for your website ranked as you want in the search results on the internet.