Learn more about security groups
Applies to: Exchange Online, Exchange Server 2013
The primary purpose of a security group is to assign permissions to Active Directory resources to a group of users instead of assigning permissions to individual users one at a time. A mail-enabled security group can be used to distribute messages to group members as well as to grant access permissions to resources. Mail-enabled security groups have most of the features and properties that distribution groups do, such as the ability to configure membership approval, delivery management, MailTip, and group delegation settings.
Consider hiding mail-enabled security groups from your organizations address book because they're typically used to assign permissions to group members and not to send email. Group members and other users can still send email to the security group by using the group’s email address.