Here are two types of serach engine optimization i heard about
1.White hat Seo
2.Black Hat Seo
Purpose of both of same then why Black hat is discourged by google ?
what are techniques invloved ?

The techniques used in black hat seems illegal that involves some manipulation and tweaking of the process. Usually the topic entered by the user into in search engine box is not the one that will come put at the result page.

It sounds like if some one searched for "xyz" and found the site which has a black hat seo ,and content on the page will be like "abc" .is not it?

That's right. And this is not the most preferred method by webmasters because your site will eventually be get deleted by search engines.

May be there will be some tools for checking it but if we do something like this in start and afte having a good page rank then changes content then is not ok?