Nofollow is an attribute used in HTML to tell the search engine to follow the links or not, to give credit to links or not. It stops crawler if you have used this attribute at a link and crawler won’t escape from that link and it won’t give page rank to that link you have nofollowed. its used like the following.
<a href="#" rel="nofollow">link</a>
If you have used external links in the site then it is wise to target blank to them and nofollow them. If you don’t use nofollow then it means that you are giving credit to the external site or link and you will increase the external link’s PR and decrease yours. What I am saying here is to nofollow some of your own pages. Yes, some pages in your blog, website do not need the Page Rank or credit or link juice or PR juice or whatever it is.
What do you think?