In clustering, failover means the capability to switch over an applicaion automatically to a standby server upon the failure or abnormal termination of the previously active application or server. If an individual application in a server cluster fails (but the node does not), the Cluster service typically tries to restart the application on the same node. If that fails, it moves the application's resources and restarts them on another node of the server cluster.
Failback is the process of restoring the application in a state of failover back to its original state (before failure). The Cluster service fails back a group using the same procedures it performs during failover. That is, the Cluster service takes all of the resources in the group offline, moves the group, and then brings all of the resources in the group online.