Search engine optimization (SEO)

This is the process of improving the visibility of a website or a web page in search engines like Google via the "natural" or un-paid ("organic" or "algorithmic") search results.

Other forms of search engine marketing (SEM) target paid listings. These links usually appear on the very top of Google results or on the side. You will see the word “ads” in small letters by these.

As an Internet marketing strategy, SEO considers how search engines work and what people search for. Search engine optimizers may offer SEO as a stand-alone service or as a part of a broader marketing campaign. Because effective SEO may require changes to the HTML source code of a web site and site content, SEO tactics may be incorporated into website development and design.

Another class of techniques, known as black hat SEO or spamdexing, uses methods such as link farms, keyword stuffing and article spinning that degrade both the relevance of search results and the quality of user-experience with search engines. Search engines look for sites that employ these techniques in order to remove them from their indices.

Web developers or web site hosting companies that guarantee placement on the first page of Google, accomplish this through black hat SEO, or grey hat SEO—methods that are not ‘technically’ considered black hat, but still violate the principle of have quality content.

A more detailed description is available at wikipedia SEO