Nice write up here:
http://aconaway.com/2008/03/18/hsrp-vs-glbp/
GLBP is a little different and runs on layer 2. Instead of one router taking all the traffic all the time, GLBP provides a mechanism to load-balance the standby IP. I’m sure you figured that out by the name, though. When configured, GLBP provides a standby IP just as HSRP does, but it also provides multiple virtual MAC addresses. When a host on the connected network sends an ARP request, one of the routers answers with the virtual MAC address. The next time a host ARPs, a different router answers with a different virtual MAC address. After all is said and done in our perfect world, you have an equal number of hosts sending traffic to each router doing GLBP via the virtual MACs (this never pans out due to the way machines ARP). If a router dies, one of the other participating routers takes over for that virtual MAC, and the host is none-the-wiser.
HTH
