Google has worked out a tag which works a tool. This tag is called a No Follow tag marked to links which are considered spam. This tag directs the spider while crawling. These Backlinks have no significance or consequences to Google Ranking or any similar benefit.
What is a Do Follow Link?
Any link which does not hold the No Follow Tag is a Do Follow Links. This are links passed by Google and which hold an eligibility to Google Ranking.
How do you know if a link is Do or No Follow?
There are plug-ins available which highlight a tag if any. Firefox also has a feature. Most websites /blogs have an option to be Do or No follow.
Does that mean No Follow Links are of no use?
While No Follow links are just links which are not registered by Google, they are not actually abandoned by SEO professionals. It is wise to keep a minimal percentage of No Follow Backlinks as a thumb rule according to some experts. But Do Follow is what you should look at.