I would like to know what is the difference between creating a group, a team and

A department is public, a team is restricted, a custom group can either. A basic user can create a team; an Admin user can create a team, department or custom group. If you use a custom group whenever possible, you can change the security setting after the group is created, providing more flexibility as your needs evolve.

If you use custom groups, you can change the security setting after the group is created.

For more on groups, please see:
forum.appian.com/.../Creating_Groups
forum.appian.com/.../Configuring_Security_for_Groups

Thanx!!