1. White Hat S.E.O-

White Hat S.E.O opts legal methods, strategies and techniques in order to optimize the websites or in other words it follows the search engine guidelines for the website optimization so for this reason white hat S.E.O is also known as ethical S.E.O.
White Hat S.E.O is mainly used to improve your business Online Reputation Management (ORM).
This is the slow process but have long lasting impact in the growth of the website on world wide web.
For example, a website which focuses on relevancy and organic ranking is considered to be optimized using White Hat S.E.O.

2. Black Hat S.E.O-

Black Hat S.E.O is opposite to White Hat S.E.O as it does not opts legal methods, strategies and techniques in order to optimize the websites or in other words it does not follows search engine rules and policies to optimize the website so for this reason it is also known as unethical S.E.O.
By performing Black Hat S.E.O user gets quick results but they are unpredictable and have short lasting growth in rankings.
There are times when search engine add your websites to the black list if you use Black Hat S.E.O practices more than White Hat S.E.O practices to optimize your website so this type should be avoided.

Black Hat S.E.O techniques:-

Link spamming is one of the major technique used for performing Black Hat S.E.O as it is getting links from the same sites, using duplicate content, using paid links and posting your website link on those websites that are not related to your website's category are all these comes under Link Spamming. Spamming implies creating a situation where you are exploiting an vulnerability to induce a unfair favorable result for yourself, thus adding the link part to link spamming is exploiting Google vulnerability to links as a quality ratified by inducing an unfair favorable result for yourself.

Keyword Stuffing is used to describe a shady technique website owners adopt in order to manipulate their site’s search ranking for a particular keyword, or set of keywords. For example: You sell gadgets, you include the word "gadgets" in your url, title and too many times in the content. Search engines like Google are smart enough to know whether the keyword is appropriately appearing on a web page or it is being stuffed.

Cloaking is another most common Black Hat S.E.O technique as some users chooses wrong keywords in order to rank their websites. for example- if someone use keywords related to hotel but their website is related to another content say gadgets so using false keywords in order to rank that gadget website this is particularly called cloaking.

Hidden links and hidden text came into existence much before Google came into the search engine world. When search engines started gaining popularity in mid 1990s, lots of aggressive S.E.O started using methods for tricking the search engines and gaining traffic. There are many types of hidden text. The most common method used is the same color background and same color font.

Article spinning is another most common Black Hat S.E.O technique as article is re write in another form i n order to prevent the duplicacy.

3. Grey Hat S.E.O-

Grey Hat S.E.O is combination of both White Hat and Black Hat S.E.O
Grey Hat S.E.O comprises of 95% White Hat S.E.O and only 5% of Black Hat S.E.O.

Grey Hat S.E.O techniques:-

You have to submit your website to the web directories to get relevant and high-quality backlinks. Not all web directories are same. So you should research and submit your website only to web directories that can generate high-quality backlinks.
Buying the expired domain is one of the most popular grey hat technique. This strategy refers to buying an expired domain to improve your website backlinks. When you buy an expired domain, you should be careful to buy a reputable URL.
When it comes to generating traffic, long content works better so you should gear up your old content so that your website rankings can be improved.
You should take the advantage of right keyword density as your content should not sound stiff because of you are doing keyword stuffing as it will hurt your search engine ranking. But, keeping the right keyword density in your content can do wonders for your rankings.