Some backlinks are inherently more valuable than others. Followed backlinks from trustworthy, popular, high-authority sites are considered the most desirable backlinks to earn, while backlinks from low-authority, potentially spammy sites are typically at the other end of the spectrum. Whether or not a link is followed (i.e. whether a site owner specifically instructs search engines to pass, or not pass, link equity) is certainly relevant, but don't entirely discount the value of nofollow links. Even just being mentioned on high-quality websites can give your brand a boost.
A backlink is a reference comparable to a citation. The quantity, quality, and relevance of backlinks for a web page are among the factors that search engines like Google evaluate in order to estimate how important the page is. PageRank calculates the score for each web page based on how all the web pages are connected among themselves, and is one of the variables that Google Search uses to determine how high a web page should go in search results. This weighting of backlinks is analogous to citation analysis of books, scholarly papers, and academic journals. A Topical PageRank has been researched and implemented as well, which gives more weight to backlinks coming from the page of a same topic as a target page.