For new players
WoW Rookie: Introduction to Reputation
New players should start with this article to learn what all the terminology means and how rep affects your gameplay.
WoW Rookie: Azeroth Reputations
This article is the next step in understanding rep and how it works for the world of Azeroth. Since you should start worrying about rep long before you hit level 40, this article shows you how to start preparing for getting your first mount -- especially if the one you hunger for is not your toon's racial mount.
Breakfast Topic: Reputation grinding
Here you can see what your fellow players have to say about what it's like to grind rep after level 60.
For new level 70s
Ask WoW Insider: Do you optimize your rep grinds?
Advice from us and other readers on how to streamline your rep grinds in Outland.
Faction frustrations eased in Wrath
A preview of how rep may change in the Wrath of the Lich King expansion.
Here are a couple of rep calculators we've reviewed.
For extra credit
WoW Rookie: Pre-Burning Crusade engame reputations
This is a good primer on how rep worked prior to the release of The Burning Crusade expansion. Rep grinding in Azeroth after level 60 is not overly relevant now, but you may still be curious as to how it worked.