Search Engine Optimization (SEO) is a set of methods, tactics to improve the number of visitors to websites from search engines listings, and not to mention in 'organic' or 'natural' listing.
SEO is an offshoot or subset of Internet Marketing/ Search Engine Marketing(SEM) . Where SEM is much wider than SEO and therefore just an element of SEM yet its an important part of SEM. Searh Engine Marketing includes the process of marketing websites by improving visibility in organic/ natural results (SERPS) of search engines, paid/ sponsered listings or combination of both, whereas SEO is just confined to organic/ natural listings.
The diagram below would make the picture clear - (Click on Image to see Clearly)
As Evolution of web and internet is much faster than evolution of man, the capability of web in terms of marketing can be estimated from the fact that Search engine optimization was started in 1996 just a decade and yet its growing and creating demand and need.
Alan Emtage, a student at the University of McGill, created the first true "search" facility in 1990 called Archie (still in use at http://archie.emnet.co.uk) , Gopher came in to existence in the next year created at the University of Minnesota as an alternative to Archie, in 1994 "Search Engines" were born in real sense these are - Galaxy, Lycos, Yahoo. Exeperimenting started in that very year to improvize and modify the way to use search engines and search engine optimization concept.
SEO Links-
Google Guidlines for SEOs
Yahoo Quality Guidlines for SEOs
MSN Guidlines for Optimizing Sites
FTC (Federal Trade Comission) letter to search engine companies -
SEM Links-
Search Engine Marketing Professional Organization
Different Strokes-
