According to the HTML protocol, there are two types of hyperlinks - these are the dofollow and the nofollow links, as have become popularised in the SEO world. Apart from their technical difference, it is good to know why they were invented in the first place, and most importantly, how to locate them.
Dofollow hyperlinks are basically the default (normal) hyperlinks which do not have the nofollow tag. Therefore if a website does not use the nofollow tag, you can treat the link as dofollow.
A normal dofollow link looks like this:
whereas a nofollow link looks like this:
<a href="http://www.r00tb0x.com" rel="nofollow">r00tb0x.com</a>
As discussed in a previous article about backlinks, search engines rank pages according to the number of links pointed at them. In the beginning all links were dofollow and spammers started abusing this technique by placing links pointed to their websites on blogs with unmoderated comments, thus increasing their page rank. As a countermeasure against spammers, the nofollow tag was invented to indicate an unapproved link - this tells the search engine to disregard the link and therefore not contribute in the page rank. Nowadays, most blog software marks comments as nofollow (irrelevant of moderated or not).
You may have already figured out, but to tell if a site is dofollow or nofollow, you can look at the source of the page and search for those tags.