[fallentimes]

In general terms:

Spam: thin site, lacks original content, few pages indexed, crawled infrequently, ranks for a small selection of keywords, weak backlink profile

Authority: many pages indexed, thick site, loads of original content, crawled frequently, ranks for a variety of keywords, diversified backlink profile

Page Rank used to be a strong indicator of this, but currently a better indicator is how often your site is crawled. That's why one authority link can do worlds more for you than hundreds (or thousands) of spammy/low quality links.

[dhimes]

I guess it's all about how you define "thin," "few", "small," and "weak." A page on how to eradicate your home of carpenter ants would possibly look like all of these, yet may be the most "authoritative" site on the web on the small, niche subject.

[joeyo]

If it is authoritative, then you would expect it would (eventually) get linked to, no?

[dhimes]

Perhaps, but not necessarily that much. How many people would put a link to how to build a dog house on their web page? Anybody who would probably has links to other dog house things (buy a book on Amazon, keep it smelling fresh with this goo from WalMart) etc., diminishing your algorithmically determined authority.

Not everybody blogs about everything going on in his/her life.