To understand policy details, you first have to find the relevant pages, next get the basic gist of a policy, and only then look at the more specific points. The precise wording of a policy may well change over time while the general idea remains the same.
For many policies, Wikipedia has handy nutshell summaries, which we've imported (current as of August 2007). For others, we've written our own summary. The uppercase abbreviated title is the page shortcut, less WP; so, for example, you can find Attack Page (ATP) at WP:ATP.
Policies fall into a few classes. Some deal with article content, and others deal with editor interactions. We've broken them down into four types for convenience.
Content policies deal with article content, both what articles should be and what you can do with them.
Social policies deal with how editors should behave and interact with one another on the site.
These are basic documents on which various processes and administrator actions rely. For example, under the Username policy (UN), accounts with unsuitable usernames will be blocked. These policies are often intended for specific situations.
This is a selective list of some guidelines we consider particularly important. There are over a hundred guidelines total, many of which are part of the Manual of Style (you can find a complete collection of guidelines at [[Category:Wikipedia guidelines]]). Summaries for interesting guidelines tend to be significantly longer than for official policies. They are often saying something important but more diffuse. They are certainly more rewarding to read casually.
The five pillars are a good place to begin to familiarize yourself with Wikipedia principles. Perhaps they will live up to the resonant name and serve as a timeless description of Wikipedia, or perhaps they'll just be eternal by Internet-time standards. Policy does evolve, and Wikipedia evolves, too. Before you say you understand the site policies, you might want another perspective. Everyday life on the site will convince you that participating is not quite so simple.
Here is our selection, based on sheer utility, of the major policies to familiarize yourself with first:
Neutrality (NPOV) from the content policies
Three-Revert Rule (3RR) and Civility (CIVIL) from the social policies
Criteria for Speedy Deletion (CSD) from the enabling policies
What Wikipedia Is Not (NOT) from the general policies
Together, these policies convey the same ideas as the five pillars but are a little more current in their emphasis. Criteria for Speedy Deletion (CSD) is now used in an aggressive fashion to clean up newly created articles that don't meet Wikipedia's standards at all. Therefore, a new editor should know about this policy.
To summarize, be a neutral, civil editor who doesn't rely on reverting pages excessively. Understand that Wikipedia provides space online for its mission to write an encyclopedia and for no other reason, and understand that many submissions of new pages will be deleted summarily from the site because they don't fit the content policies.
That's five policies. Two guidelines may also affect you as soon as you start editing: Conflict of Interest (COI) and Reliable Sources (RS). For obvious reasons, not every guideline can be covered in detail, but these two are very important.