I heard somewhere that clocking and doorway pages are blackhat seo technique and i had read more about it but i did get it so please tell me about those.
Cloaking: Delivering one version of a page to one visitor and another version to others.
doorway pages : Doorway pages are Web pages designed and built specifically to draw search engine visitors to your website. They are standalone pages designed only to act as doorways to your site.
These both are black hat SEO techniques.
The doorway page is highly optimized with hidden links and keywords that never sees users. It optimized for search engine for ranking and all the traffic re-direct to other poorly optimized site.
Cloacking technique also a dirty trick of SEO for search engine rankings. Using keyword stuffing technique, optimize the site which is invisible for normal web users but seen by a search engine spider. It is not a good practice of SEO.
The term is "cloaking", not "clocking".
Cloaking is a way of giving different content to visitors and search engines. This means that you can give real people a useful website, and search engines a keyword-stuffed page to con them into thinking your page does more than it really does.
Not surprisingly, search engines aren't too keen on this, and any sites they find using cloaking techniques are likely to be completely blocked. In other words, don't do it!