Shaddows - 9:57 am on Nov 6, 2012 (gmt 0)
For a given value of "understand"
In broad strokes, Google is pretty simple. In detail, it's incomprehensively complex.
The SERP-generating part of Google doesn't like too many ads, doesn't like autogenerated, spun, or syndicated content, and wants searchers to be happy with BOTH the mix of results on the SERP, AND the quality of the site they eventually visit.
They penalise too many ads, overt manipulation (of many sorts) and repeated violation of quality quidelines.
It IS hard to get on to the first page of results, simply because of maths. For a conservative million-result SERP, your page has to be in the top 0.00001% of all results. Being merely the in the top 0.001% means page 100 obscurity.