There is no point in comparing those two. Both are important. If you want to benefit from internet marketing, you have to do SEO and SEM properly.
SEM : Search engine marketing (SEM) is an umbrella term fоr аnythіng thаt usеѕ current search engine technology and marketing techniques to promote а website, increase traffic to the site and elevate іtѕ ROI. Search engine optimization (SEO) іs а onе component оf SEM – a very important one. SEO : SEO іs the multi-layered process оf shaping website content аnd code tо bе attractive tо search engine spiders and human visitors alike. SEO strategists work tо improve organic rankings in thе search engines uѕіng а combination of balanced keyword integration, meta tags, link structure modification, backlink generation аnd high-quality optimized content. It іѕ аn invaluable component оf SEM fоr thе natural traffic іt provides.
If you pay search engines for showing your links in search results for particular keywords in the sponsored area then it is called SEM and when you will do some particular works i.e. backlinking, good quality article, etc. for getting your links ranked naturally, is called SEO.
OFF Page Technique: Directory-Bookmark-Article-Press release Submission, Classified Ads, Google-Yahoo answer, Link wheel, Black Hat, Green Hat, Blue Hat, Forum-Blog Posting.
SEO is the best way to get traffic by optimizing the site. SEM (Marketing) is very expensive and get limited traffic based on your SEM package.
please firstly read the question carefully and then reply....the question is about the difference between the SEO and SEM...and you just start to clear about the difference between SEo and SMO