Search Engine Optimization (SEO) attempts to achieve greater visibility for a website by ranking well within search engines.
Search Engines
Search engines such as Google or Yahoo are used by millions of people every day to find information or businesses. According to Quantcast, Google receives over 214 million monthly visits, almost double the traffic of its two main competitors (Bing and Yahoo).
Search Engine Algorithms
In order to maintain their viability, search engines strive to provide the most relevant results by ranking websites according to their own proprietary algorithms. Generally search engines do not divulge their ranking algorithms; however, they do sometimes provide guidance to help website owners. For example even though Google largely values the number and quality of inbound links, Google advises website owners to create websites with great content which will in turn generate natural links from other websites. To prevent people from exploiting their algorithm, Google actively identifies and penalizes link networks (Anglo Rank, Ghost Rank, SAPE, etc) and websites that use unnatural links (such as JC Penney in 2011).
Ranking Up
Although search engines rarely publicize specifics of their algorithms, good SEO professionals learn the basics of how search engines work and have a general idea of many factors which can positively affect a website's search engine rankings by studying analytics, researching competitors, and staying current with SEO news and the SEO community.