While this is an issue that has touched many times in SEO blogs, I will try to answer (in general) to a question I was asked a while ago through Twitter.

The best sources of links remain the links from high PageRank sites and topics related to ours, especially if they are old domains. This is what is known as quality links, which are more difficult to obtain.

Incoming links from social media are taking a new weight, especially if they have nofollow. In this case, we can use social bookmarking as an alternative to create your own backlinks.

Regarding Trashy links are those that correspond to sites that have nothing to do with ours, to directories that require reciprocal links or link farms.

Directories with good PageRank, related to our site and do not require reciprocal links or prestigious forums can also serve linkbuilding to work on our blog.