A backlink is a link from a site to another resource located on the Internet. We use backlinks to reference content, just like we refer to books for example.
Backlinks are important because they are a sign of authority. When a website has many backlinks (quality is important as well, as we will explain below), it is seen as being more credible and trustworthy than a website that doesn't have many backlinks. Search engines use this information to rank websites in their search results.
Backlinks can come from many different sources. These include social media posts, blog posts, guest posts, and other websites. Many businesses also engage in link building activities looking to gain more backlinks.
It is important to note that not all backlinks are created equal. Some backlinks are more valuable than others. Quality backlinks come from authoritative websites and have a higher chance of improving a website's search engine presence and ranking.
Backlinks linked from spammy websites are less valuable and can even harm a website's ranking. It is important to create backlinks from reputable websites and to avoid creating backlinks from websites that are considered to be low quality.
Links may also mean an endorsement to the linked page, but sometimes you do not want to endorse linked resource, specially for SEO reasons.
HTML attribute rel defines the relationship between linked resource and the current document. From SEO perspective, we use these
We use nofollow to indicate that the current document's original author or publisher does not endorse the referenced document. We use this tag in cases, where we want to reference given resource, but don't want to pass "link juice" to it.
Typical use for nofollow backlinks is for links where we don't have control over the referenced content and where links could be exploited for SPAM purposes etc. Example: comments on blogs.
UGC stands for User Generated Content. Similar as with Nofollow, Google now has specific tag for user generated content that we, webmasters do not have full control and link on our site (for example from comments) doesn't mean owners endorsement.
This tag means the resource is outside the site it's linked from - external link. It does not instruct SE to nofollow the link though.