Search Engine Optimization is the process of improving the visibility of a website in organic (UN-paid) search engine result page by adding search engine friendly elements into a website.
There are mainly two types of SEO:
White hat SEO: White hat SEO is a technique that utilizes the strategies that do not violate the search engine guidelines (mainly Google). This technique is used to achieve results in a natural way.
Black hat SEO: Black hat SEO comprises with the strategies and techniques that are used to get high rankings exploiting the weaknesses of the search engine algorithms in a short period of time, such techniques violate the guidelines of search engine.
Now a day’s one more type of SEO is becoming popular i.e. Grey hat SEO
Grey hat SEO: Grey hat SEO is somewhere in between White hat and Black hat SEO, it can also be said that using Grey hat strategy is like playing with a fire unless you are not sure about the after effects of your technique.