If a firewall is a gate, a session border controller is a canal. Whereas a gate can only be opened or shut, canals have a series of trenches which are filled and then released. This buffer allows much more complex checks and adjustments than a simple open or shut gate. A Session Border Controller has some of the layer 2-4 port controls, but where they really shine is in their Layer 7 capabilities.

via 2600hz Blog • The Difference Between a Firewall and an SBC.