A device group is a component of a device service cluster. It consists of a collection of BIG-IP devices that trust each other and can synchronize, and sometimes fail over, their configuration data.
device service cluster
Device Service Clustering provides synchronization and failover of BIG-IP configuration data among multiple BIG-IP devices on a network. You can configure a BIG-IP device on a network to synchronize some or all of its configuration data with other BIG-IP devices; fail over to another available device; and/or mirror connections to a peer device to prevent interruption in service during failover.