You probably should be reading the Pacemaker site clusterlabs.org
This site conserves Heartbeat specific stuff. See Site news for details.
Fencing is the process of locking resources away from a node whose status is uncertain.
There are a variety of fencing techniques available.
One can either fence nodes - using Node Fencing, or fence resources using Resource Fencing. Some types of resources are Self Fencing Resources, and some aren't damaged by simultaneous use, and don't require fencing at all.