The Configuration API is a set of tools to help developers quickly parse and emit configuration files that are human readable and editable. Despite the name, the API can easily be used to store plugin data in addition to plugin configuration. Presently only YAML configurations can be used. The API however was designed to be extensible and allow for other implementations.
The Configuration API can be found in the org.bukkit.configuration and org.bukkit.configuration.file packages. Plugins that were created before version 1.1-R5 may have used an older and different implmention that resided in org.bukkit.util.configuration. These implementations are not compatible and the old package has since been removed.
This introduction assumes that you have some knowledge about proper object oriented design, Java, and the core design of Bukkit plugins. This page is not a substitute for the JavaDocs for the FileConfiguration Class