Packs vs Dens
Each Cub Scout Pack is comprised of multiple Dens, based on age group.
Dens are named:
Kindergarten is Lion,
First Grade is Tiger,
Second Grade is Wolf,
Third Grade is Bear,
Fourth Grade is Webelos I, and
Fifth Grade is Webelos II (or Arrow of Light)
The Dens are also numbered, and in some years there may be more than one Den for each grade.
The typical Den size tends to be 6-10 Cubs.
Each grade (Den) works throughout the year on achievements from their respective handbook with age appropriate activities and requirements.
In Fifth Grade, the Webelos II Cub Scouts cross over into Boy Scouts (January) where they become part of a Troop and Patrol instead of Packs and Dens.
The Dens meet individually on their achievements but the Pack will also meet regularly for larger group events.
For more information about the time commitment involved in Darien Cub Scouts, see this link.
For more information about Scouting overall, see the Scouting 101 pages.