Difference between SEO, SMO, SEM:

1. SEM is a much broader term than SEO, and actually includes SEO as one of its techniques. SEM is all that a company can do to advertise itself on a search engine, including paid inclusion and other ads.
   
   Search engine marketing (SEM) is the act of marketing a website via search engines, whether by improving rank in organic or natural listings, paid or sponsored listings or a combination of both.
   
   Social media optimization (SMO) Social Media Optimization is the methodization of social media activity with the intent of attracting unique visitors to website content.

2. SEO - Is optimizing the page to be easily found and index able by search engines. This is everything from content writing to getting backlinks.
   
   SEM - Is bidding on keywords through tools like Google AdWords. This can be extremely expensive and time consuming, and should probably only be a short term solution until your site is performing well organically.
   
   SMO - Is setting up and participating in various Social Media sites, like Facebook, Digg, Delicious, etc. The purpose here is to build communities of fans and spread the word on your updated content.

Definitions:

SEO: Search Engine Optimization

In simple words, we can define SEO as improving your rank in search engine result listings on Google, Yahoo and Bing. Often you will hear the terms “organic” or “natural” search results which refer to the areas of the results pages that are not “sponsored” or paid for advertising. Actions to improve SEO include keyword optimization, relevant content creation, creating directory listings, and much more.

SEM: Search Engine Marketing

The easiest distinction between SEO and SEM is if that the place the link shown is paid for, its SEM. SEM refers to ads and links that are “sponsored” to be placed in designated sections of results pages. If you are participating in paid advertising on a search engine, your ad may show within the special sections of the page reserved for advertisements. Another major distinction, when you “turn off” your SEM campaign, your ads will stop showing where as SEO content is more permanent.

SMO: Social Media Optimization

Social media optimization is choosing the best options of what is available to improve your use of social media. The goals of social media optimization overlap with SEO and SEM. Different from SMM (social media marketing) which is more implementation and advertising in paid sections within the social media platforms, SMO is the analysis and strategy creation of what platforms (Facebook, Twitter, etc.) are and/or should be used and how they are being used.