Linktypes

People here always tell to disverify your backlinks but...

....where is the difference in social bookmarking and submissions to website directories?

....where is the difference between article submissions and web2.0 submissions and "Guestposts" on other blogs?

Where is the difference from a wordpress blog with 1 Million articles and an article directory like ezines?

Are these terms classifing more things like Outgoing links, indexed sites, etc.? So is a wordpress website with 1 Million articles (with 2 outgoing links each) more an article directory or more a blog?

It just looks more natural to the Search Engines if you have extreme link diversity. Let's say you have 100% profile backlinks for your site. You'll get hit by the panda and you won't get ranked as high as you would if you had backlinks from forums posts, blog posts, articles, etc.

No one can tell you exactly which method works the best so why not get backlinks from every single type of site out there

There is no difference. Do them all for maximum link diversity.

some are nofollow...

There is no inherent difference. But search algos based on link evaluation try to introduce a diference which at least in the beginning was done manually. So in those cases it is not just Mainpages with Subpages. Some are MAINPAGES with subpages, others are mainpages with SUBPAGES etc.; capital letters indicate link weight.

This is the crude definition of pagerank (the internal, secret metric, not the publicly viewable PR)

Now, there are specualtions that other, auxiliary algos also exist that try to guess if the links were gained 'naturally' or it is a trick by the webmaster. Proponents asume that independent linking would be a chaotic mixture, hence the advice to try to mimic that chaos, which also includes mixing 'click', 'here', 'keyword' etc. as anchor. Whether a webmaster decides to agree with this auxiliary algo hypothesis is up to him.

As to the pagerank itself, it is not a hypothesis, it is the real deal. The wikipedia article explains it pretty well, have a look at the image near the top.
http://en.wikipedia.org/wiki/Page_rank

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Edit & Addendum:
Personally I don't care about diversification. I go for (public) PR 6 and above and try my best to put a backlink there with an exact match anchor text.

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