SEO - Search Engine Optimisation is a technique to improve the ranking of your website. Its a generic(unpaid)marketing discipline focused on your Website. This technique is useful and stable. All search engines like google, yahoo, bing, etc. have their own indexing system. In SEO we work with content and images on our website, this will help us to index our URL with these search engines. We use some more techniques for this like submitting our URL in directories.
SEO has two types one is a black hat technique and another is white hat. This white hat technique is always preferred to improve ranking. This technique is suggested because it improves our ranking gradually, but surely. On another hand, the black hat technique is not good, because it has some issues which are not suggested by search engines and some times this leads to downfall your ranking.