When you switch on the private messaging system, members will be able to contact each other without email. You frequently encounter this sort of internal messaging on discussion forums.

Once you have the switch on, you can also choose which usergroups can use private messaging (see Admin -> Members -> Manage Usergroups).

In the member profile there's an option to select to accept private messages from everyone, no one, or just from buddies if the buddy list switch is enabled.