BPDU Guard takes effect only on portfast ports. You can therefore think of BPDU guard the same as portfast BPDU guard when a port is a portfast port.
PortFast BPDU guard can prevent loops by moving a nontrunking port into the errdisable state when a BPDU is received on that port. When the BPDU guard feature is enabled on the switch, spanning tree shuts down PortFast-configured interfaces that receive BPDUs, rather than putting them into the spanning tree blocking state. In a valid configuration, PortFast-configured interfaces do not receive BPDUs. Reception of a BPDU by a PortFast-configured interface signals an invalid configuration, such as connection of an unauthorized device. The BPDU guard feature provides a secure response to invalid configurations, because the administrator must manually put the interface back in service.
When enabled on the switch, spanning tree applies the PortFast BPDU guard
feature to all PortFast-configured interfaces.
Portfast BPDU guard can be enabled or disabled on a global basis, thus
affecting all ports with portfast configured.
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a008009482f.shtml
