“Long Tail” is a term for a new marketing theory coined in 2004 by “Wired” editor Chris Anderson.

The world, theorizes, Anderson, is divided into a few marketing entities that are dependent on monster hits (think Prius or MI:3).  But most companies (and arguably non-profits) are more dependent on niche audiences, who are efficiently reachable online.

Read this Time magazine summary — it explains it better than I ever could.
The Bottom Line is this: if you’re not Nature Conservancy or the Red Cross, you better understand the long tail.