"Group applications are new kinds of applications within Yahoo!
Groups that allow owners and moderators to customize applications
specifically for the needs of their group."

ie

Address Book
A dynamic, online address book that allows group members to stay in
touch.

For Sale
A great way to buy and sell with other group members.

Give Away
An easy way to give stuff away or find items for free.

Jobs
An easy way to post jobs to the group.

People Map
See where other group members live and share where you are from. But
only share as much as you are comfortable with.

Places Map
A great way for group members to share information about places that
are relevant to the group.