If you are a web surfer with even the slightest degree of familiarity regarding the mechanics of the Internet market, then you certainly must have encountered terms like SEO (Search Engine Optimization) and SEM (Search Engine Marketing), respectively, at some point in time. The thing is that, in the absence of proper knowledge, these two concepts have long been mystified and mixed up and are still not fully understood, despite being extensively (although often erroneously) used in many different contexts. 

Probably the main reason why most people seem to mistake Search Engine Optimization for Search Engine Marketing and the other way around is that the two concepts are in fact closely related and interwoven concepts. Before we jump in to discuss the basics of these powerful Internet marketing techniques, let’s sketch out the corresponding definitions first and try to briefly learn to separate between the two terms.

SEM refers to the practice of boosting the prominence of websites by increasing their exposure in search engine listings via advertisement implementation (mostly contextual advertising through sponsored links), paid inclusion (search engine services that charge companies for website inclusion in their search index) and by making full use of Search Engine Optimization, or SEO.

Standing out as a characteristic subsection for the aforementioned concept, Search Engine Optimization (SEO) comprises the set of techniques implemented to increase the quantity and/ or quality of Internet traffic to a particular online resource (website, blog, etc.) by addressing all aspects necessary for achieving higher rankings in search-engine results.

What should be remembered is that SEO is generally a more technical and action-oriented process, while SEM places the emphasis on elaborate planning. Therefore, the Search Engine Optimization professional must possess a better level of expertise in terms of web designing, as opposed to a Search Engine Marketing professional, who is required to have a more comprehensive understanding of the online advertising field. 

Concerning specific technical requirements, SEO professionals must be well versed in areas like web designing, programming and software, as well as hold the grasp over all the subtleties of search engine technology. At the opposite pole, SEM experts should focus mainly on segments like advertising and marketing.      
The degree of direct tangency with search engine management and administration further deepens the gap between Search Engine Optimization and Search Engine Marketing. While SEO professionals rarely (or never) need to get involved in search engine administration processes, SEM professionals are required to directly handle search engine administration issues, considering they may be needed to manage the display of advertisements, analyze and determine costs, etc.  

In essence, you should keep in mind that the main professional role of Search Engine Marketing experts is managerial and business level decision making, as opposed to Search Engine Optimization experts, whose responsibility is to deal with the technical and scientific aspects of marketing.