Link building is a way to check how many sites link to your site

It can be of different types like

1 - Oneway link building - Your link is placed on other web page ( The best method for SERP

2 - Reciprocal/Two way link building - You have to exchange the link by placing the link on your page.

3 - Three way link exchange - site 1 links to site 2 , site 2 to site 3 and site 3 to site 1.. So indirectly it fools search engines.

You can buy/sell links from websites like text-link-ads.com or adbrite.com

You can do link exchange from sites link linkmarket.net, linkalizer.com and so on..

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