People are often confused when the official USGBC website sends LEED candidates to the GBCI website. That's because USGBC creates the reference guides, rating systems and education programs but oursources the actual "testing" to GBCI (Green Building Certification Institute). This allows USGBC to concentrate on continously updating and creating new green building ciruculum.
Take a look at the chart below for a visual overview.
For more information on GBCI, visit the GBCI Website