As a Less Wrong account holder you may cast a single vote on articles and comments. You may choose to vote up or vote down. An up vote adds 1 points to the article's or comment's score, and also 10 (for an article) or 1 (for a comment) karma points to the author. A down vote subtracts 1 points from the article (or comment), and also 10 (for an article) or 1 (for a comment) points from the author's karma. The specific guidelines around casting votes has been a topic of discussion on Less Wrong.
Note: The number of down votes you may cast is limited to 4 times your karma value. E.g. If your karma is 23 you may cast up to 92 down votes.
In order to post an article in the main section of Less Wrong, a user needs 20 points of Karma. To post in the Discussion section, one needs 1 point.
Articles are selected for promotion on the basis of substantive new content, clear argument, good writing, popularity, and importance. Promoted articles appear on the Less Wrong home page. Non-promoted articles are still available in the other views, such as New or Popular.
Moderation of the blog is primary maintained by use of the voting/karma system. Moderators exist primarily to deal with spam, but there are other cases when posts and comments can be deleted, as detailed in the Deletion policy. Editors correct poor formatting. Eliezer Yudkowsky promotes articles.
There is also a "Ban" button below each comment (if you have the necessary permission) which makes the individual comment invisible to non-moderators.
(Technically, Main and Discussion are considered to be two independent forums, so you have to add moderators and ban users at each of them independently.)
Articles may be edited for non-standard markup (fonts, indentation, etc., which usually result from copy-pasting from an editor); missing summary breaks (in meetup posts these should be placed at the very beginning); empty lines (paragraphs implemented using <br /><br /> instead of </p><p>); isolated clearly unintended typos; errors in URLs where it's clear how to fix them (e.g. some spurious prefix or suffix made it in the URL, or the content is no longer available, but a mirror site exists); missing spaces around markup (happens with URLs or italics sometimes); extra letter included under or missing from linked text string and to fix tags (especially but not only) on standard post types (quotes, open threads, meetups, posts in new user sequences). Editors shouldn't fix bad wording, systematic typos, or non-standard formatting that was intentional and looks fine. Editors may do more than these specified tasks if solicited by post authors.
Changes made by Editors are visible at http://lesswrong.com/edits.