A DHCP Spoof attack is a illegal DHCP server setup in a LAN environment which configures the clients with wrong IP information. The illegal DHCP server can configure the clients with it self as the default gateway, in which case all non-local traffic will be sent to the illegal DHCP server where the intruder can inspect or alter the packets before they are sent on to the real gateway. this is known as a man in the middle attack.

Protecting against DHCP spoof attack

On the access switches you can protect against spoofing

ip dhcp spoofing
interface GigabitEthernet0/1
 description Trunk to Distribution layer and DHCP server
 ip dhcp snooping trust

See Netband Project - DHCP Snooping