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First its very important to understand that what are backlinks, because a lot of people do not know about backlinks although they are keep hearing word "backlink" in advertsing and from people talking about backlinks how to increase etc. So backlinks are links to your site on other websites , so it make one kind of network that too how many other website you are connected and to your website how many other sites are connected this make one network so that bigger is the network or can say the higher is the number of backlinks and its better for your site :), and so your site will be ranked higher in search engines and you will get high volume of traffic. So in SEO backlinks are like backbone its very important to make backlinks and remove unwanted backlinks because sometime spammy or adult site backlink can be bad , so its better to have backlinks from clean well reputed website.
Useotools onlline backlinks checker is free premium quality seo tools that can check backlinks to your site , so that you know how many website are linked to my website. Usage of this tool is very simple , you have just to enter your website URL and then validate captcha and then submit and then you will see number of backlinks your website have.