Online Visibility

SEO:
search engine optimization is the process of improving the quality and volume of web traffic to a website by employing a series of proven SEO techniques that help a website achieve a higher ranking with the major search engines when certain keywords and phrases are put in the search field.
http://www.brickmarketing.com/define-search-engine-optimization.htm

SEM:
Le Search Engine Marketing est une activité visant à optimiser sa présence sur les moteurs de recherches par référencement payant (liens sponsorisés) ou naturel (encore appelé Search Engine Optimization – SEO). Son objectif est de placer un site Internet parmi les premiers résultats des moteurs (Google, Yahoo, MSN, Baïdu) sur les recherches liées à son domaine d’activité direct ou indirect.
http://www.marketing-digital.fr/2007/03/definition-du-search-engine-marketing/

Pay Per Click:
A brief Pay Per Click definition is: PPC is a type of sponsored online advertising that is used on a wide range of websites, including search engines, where the advertiser only pays if a web user clicks on their ad. Hence the title, ‘pay per click.’
http://www.brickmarketing.com/what-is-pay-per-click.htm

Social Search engine: 
A social search engine is an enhanced version of a search engine that combines traditionalalgorithm -driven technology with online community filtering to produce highly personalized results. A few social search engines depend only on online communities. Depending on the feature-set of a particular search engine, these results may then be saved and added to community search results, further improving the relevance of results for future searches of that keyword.
http://whatis.techtarget.com/definition/social-search-engine

Organic Search:
An organic search is a search that generates results that were not paid advertisements. Many customers feel that these results are a more accurate reflection of what they might want when searching for a specific phrase, because they are generated by popularity and common usage.
http://webdesign.about.com/od/seo/g/bldeforganicsea.htm