All links that are dofollow, passes a bit of link juice with it. It's kind of value flow from the host site to the site that gets links. It works much like recommendation in real life.
The down side of link juice is that when too much much of out going links are added, it result in the drop of link value of the host site. Adding no follow attribute to links will prevent passage of link juice.
Having said all that, links and link juice are not as much a subject to worry as it once used to be... Since PR itself has become minor factor in SEO today.