Often, especially in a forum such as this, people leave backlinks to websites that are not related to their site's content. Is this useful? Is a dofollow backlink with proper anchor text from a site that is not related better than no backlink at all? If so, what are the pros/cons for doing this? For instance, in my signature there are links to my site, but it has nothing to do with SEO. Does this hurt my backlink to the point that my signature is hurting my sites's SERP and PR?
Hi, the key moment here is the quality of the sites which link to your websites - the higher is their PR - the better is the quality of incoming links to your website.
Yes, I think Quality of the incoming sites is more important than the quantity. A site with good page rank and having a dofollow is what gets you more rank in SERP's.