A nofollow is a hyperlink attribute which requests search engines not to increase the importance of a visiting site specified by that hyperlink in it’s index and thereby having no change on the overall rank of the link target.
This was primarily devised to reduce spam and maintain the overall search quality of the search engines but for building backlinks this doesn’t help much. [On most websites which have the comments section links as nofollow,visiting them and leaving a web address back to your blog/site wouldn’t have much impact in increasing the overall value of your own site/blog because of nofollow attribute.]
By the way,the comments on this blog are all dofollow because they are moderated before publishing thereby combating spam.
Once installed,simply right click on a website and select NoDoFollow to find if links are nofollow or dofollow,if they are no-follow then they will be highlighted in pinkish red,for dofollow in blue.
Simple and efficient.