Cisco Cram or: How I Learned to Stop Lolling and Love the Book: CCIE 400-101: Layer 2 Technologies

Loop Guard

When a redundant port no longer receives BPDUs, the STP conceives that the topology is loop free. Eventually, the blocking port (alternate or backup port) becomes designated and moves to a forwarding state. This situation creates a loop.
The loop guard feature makes additional checks.
If BPDUs are not received on a non-designated port, and loop guard is enabled, that port is moved into the STP loop-inconsistent blocking state, instead of the listening / learning / forwarding state.
Once the BPDU is received on a port in a loop-inconsistent STP state, the port transitions into another STP state. According to the received BPDU, this means that the recovery is automatic and intervention is not necessary.
Loop guard must be enabled on the non-designated ports (more precisely, on root and alternate ports) for all possible combinations of active topologies.
Loop guard can only be enabled on point-to-point links.
Loop guard does not work with PortFast or root guard.
Loop guard and UDLD functionality overlap.
Both protect against STP failures caused by unidirectional links.
The highest level of protection is provided when both UDLD and loop guard are enabled.
Enable with the spanning-tree guard loop command.

The administrator can set the root bridge priority to 0 in an effort to secure the root bridge position. But there is no guarantee against a bridge with a priority of 0 and a lower MAC address.
The root guard feature provides a way to enforce the root bridge placement in the network.
The root guard ensures that the port on which root guard is enabled is the designated port.
If the bridge receives superior BPDUs on a root guard-enabled port, root guard moves this port to a root-inconsistent STP state, which is effectively equal to a listening state. No traffic is forwarded across this port.
Root guard is mutually exclusive with loop guard.
Root guard is used on designated ports, and it does not allow the port to become non-designated. Loop guard works on non-desginated ports and does not allow the port to become designated through the expiration of MaxAge.
When loop guard is configured on the port, it disables root guard configured on the same port.
You must enable root guard (spanning-tree guard root) on all ports where the root bridge should not appear.