Search Engine Optimization, shortly called as SEO, is a process of showing the search engines (like Google, Yahoo, Bing etc) that your site is a good one and offers good information to the visitors. Since search engines have some limitations to themselves, they list some guidelines for you as to how best you can optimize your sites and what are the best practices to follow to make your site search engine friendly and things of that sort. But these days the meaning of SEO has been changed by all those greedy webmasters out there, whose sole aim is to get top rankings in SERPs there by earn good revenue via their websites.
There are two kinds in SEO, one is On-page optimization and the second is Off-page optimization. On-page optimization deals with the code (structure) of your site, quality content, the title tags, meta description tags, keywords etc. While Off-page optimization is about getting quality and topic related backlinks to your site. Normally, the backlinks should be natural but these days over 90% of webmasters are buying paid links to creep up in SERPs. This way they are able to defeat the main purpose of a search engines which is to provide quality search results to their visitors. Of course, the search engines also have to blame themselves for this; because of their outdated algorithms to determine search results.
The good and bad of SEO is this: 1. White hat SEO and 2. Black hat SEO. There are also some other kinds of SEO namely, Grey hat SEO and Blue hat SEO, but these are not that important for the time being. White hat SEO is which uses search engine recommended SEO while Black hat SEO is which uses some deceiving and cheating tricks to get instant boost in SERPs. I will continue the article later but in the mean time use this link to learn more about how SEO changed over the years.