On previously I have told you about Importance of backlinks, how to create backlinks and about golden rules you need to follow. As you know backlinks have 2 type which known as "Do-follow" and "Rel NoFollow" backlinks, here example link for rel Nofollow and Dofollow :
Do-follow : <a href="Example.com">Your Keyword Here</a> No-Follow : <a href="Example.com" rel="nofollow">Your Keyword Here</a>
To see if that link do-follow or rel nofollow you can open website source code page and look on your links or you can use special add-ons for Mozilla FireFox browser NoDofollow Add-ons for free, and when you active the NoDoFollow Add-ons you will see 2 colors in your Mozilla FireFox browser, which: Red Color for Nofollow and Blue for Dofollow.
Dofollow Backlinks - as you see Dofollow means is to following or follow and that mean if do-follow backlinks mean a links that will be indexed and crawled by search engine Bots to the destination (your web pages).
Rel Nofollow Backlinks - unlike Dofollow links, the Nofollow links giving a signal for search engine bots to not crawling but indexing them, Rel Nofollow developed by Google Search to fighting against pagerank sculpting, Spammer and ranking manipulation on search engine result.
Until today people keep telling that DoFollow Backlinks better than Nofollow, because :
well, as far as I know Google Search never said that Rel Nofollow Backlinks are useless but it's up to you.