Many people will leave links on a blogs comment section or forums with a link to their site, but won't leave any valuable content to the readers, hence SPAM, so by using that no-follow, they are essentially denying the spammer any opportunity to gain the benefits link. So any link which is not a nofollow is safely assumed to be a do-follow
Do-follow links are also links that give pr juice and are considered from the search engines as links valuable for the formation of the SERP and the PR. There are whole seo strategies that are built around the idea of having as much quality do-follow links as possible.
