Backlinks are the incoming hyperlinks from one webpage to another website. They are considered good for SEO, Page Rank, Traffic and reputation of any website.
One can build backlinks by guest posting, commenting on other websites, sharing articles on social bookmarking websites. But the best thing you can do is write good quality articles and others will themselves link to you.
Finding backlinks of any website:
There are various services that allows you to find backlinks of any website. Like backlinkwatch.com allows you to check backlinks, it displays you the list of all web-pages that are linking to your website.
It is a free service and no sign-up is required. You simply need to type in or paste the domain name, hit ENTER and it will display you the results.
This is a free backlink checker service but works very well. In-case if you like fetching more details then there are various other premium backlink checker tools such as SEMRUSH and AHrefs etc.