When you have a hierarchy like Year -> Quarter -> Month -> Week -> Day, you will be using Drill down groups where the highest level is Year and the drill through function stops at Day.
In a cyclic group, you can arrange non-hierarchical dimensions as a group. Dim X -> Dim Y-> Dim Z
The cyclic group here goes from Dim X to Dim Y, then from Dim Y to Dim Z and later on from Dim Z back to Dim X. Just as the name its a cyclic process.
