Site backlinks refers, when someone put your site link or URL on his website as a useful reference..
When you put your site links/URL on external sites whether it is forum/blog/local directories etc.
Both above conditions point to backlinks.
Contributing on external sites is the most trendy and better way instead of blog commenting.
Otherwise community forum is also a great way to talk about your services and product.
Note: But keep in mind, avoid spamming activities around discussion boards and blogs. Because site administrators can easily throw out you from their community. So before join any community you first priority should be read their FAQs or polices to prevent spamming from your side.
Hope this helps!