It's all about creating accounts on as many websites as you can and loading up your profile with links to your sites. It was all the rage about 2 years ago - these days internet marketers are scrambling to delete those accounts as they are not considered spam.
It's still effective, but only on if you're posting regularly on quality forums relevant to your niche. I own a medium size forum and trust me, if I post a hundred or so profile links at a page (internal or external) (for low to medium competition keywords,) I can usually get it in on page 1 of Google without building any other backlinks. Not that I do that ....anymore.
Your profile should be mature and trusted else administrators will delete those profile accounts. You can use them for link building on Youtube videos.
This is a spam way to get links. Most forums use are typical and some time ago many black-hat seo services spammed forums with fake users in just one aim - link from a profile. No benefit for seo.
This is considered a SPAM backlink... Yes, it was working maybe a few years ago, but now you can just ruin your MAIN website... If you do it as a tier2 or even tier3 backlinks, you can try, but do not do it to your main website... It will just not work!