[hwang]

Actually it's like tag that only google bots understand whereby the main purpose for it is to prevent spam in the first place. The dofollow means that whatever link which is there will allow the bots to go beyond and continue to follow to the sites. On the other hand, the nofollow will stop the bots from going further.
How do you find out whether it's a dofollow or nofollow? Just observe the html codes. I think other online resource can explain this in more detail.