SEO stands for Search Engine Optimization and is defined as (in my own words):
"The process of finding out the best keywords for a web site and by the use of optimizing the web site along with other off-page work making that web site attain a higher position in the search engine result pages (SERPs) for those selected words."
Although the exact calculations used by the search engines are kept secret, there is lot of knowledge and observations in this field from thousands of webmasters worldwide.
It could be said to be a branch of online marketing. In general terms you can say that it means to make a web site more visible and make it look important in the eyes of search engines.
Not being familiar with SEO and not applying it compared to actually doing the right things can make a huge difference in terms of visitors to your web site.
