TreeBehavior (class)
Class TreeBehavior
Makes the table to which this is attached to behave like a nested set and provides methods for managing and retrieving information out of the derived hierarchical structure.
Tables attaching this behavior are required to have a column referencing the parent row, and two other numeric columns (lft and rght) where the implicit order will be cached.
For more information on what is a nested set and a how it works refer to http://www.sitepoint.com/hierarchical-data-database-2/