This is the good one I found on one of the MVP blog:
Clustering terms made easy
Clusters are Highly Available and should never be considered Fault Tolerant.
Highly Available = is when I come anytime my wife calls me.
Fault Tolerant = Marriage.
You don't want to be married to you SQL/Exchange Cluster :)
You do want it around whenever you need it though.
Active/Active = when your cluster is too busy for its own good.
Active/Passive = one worker, one manager, you decide which is which.
Node = Clustered computer, could also be the worker who sits in a cube, not to be confused with Dude.
Virtual Server = this is kind of like be on a telecom at work, only you are calling in from Hawaii and nobody knows.
Quorum = Cluster=Quorum, Quorum=Clustering.
Failover = the only time at work that you can fail and still be a hero.
Failback = great way to get fired, let your server failover without you controlling it (Don't confuse with the above term).
Cluster = when it fails, also known as a Cluster Fork, only fork is spelled funny - u c what I mean?