In short, a recovery ride is any ride spent in Level 1 also known as "Active Recovery". Coggan defines this level as <= 55% of your power at lactate threshold or, for those people training with heart rate monitors, less than or equal to 68% of lactate threshold heart rate. He goes on to describe this zone as:
"Easy spinning" or "light pedal pressure", i.e., very low level exercise, too low in and of itself to induce significant physiological adaptations. Minimal sensation of leg effort/fatigue. Requires no concentration to maintain pace, and continuous conversation possible. Typically used for active recovery after strenuous training days (or races), between interval efforts, or for socializing.The article continues to expand on all of the different training zones, describes the types of workouts that stress the different zones, and provides a chart of the physiological/performance impacts of working out in such zones.
My search also took me to the article James sent out last week on post ride recovery. This article provides a fantastic description of the type of fatigue we cyclists will experience as well as proper nutritional guidelines for recovering from this fatigue. For your reference, I have linked this article here.
Finally, I found a bike forum where other riders discuss the importance of being disciplined while on a recovery ride.