SharePoint server architecture in both Microsoft Office
SharePoint (MOSS) and Windows SharePoint Services (WSS) lets you
create a robust, fault-tolerant, and highly available SharePoint
farm designed to survive the loss of any one component. But it’s
not readily obvious how to do this out of the box, and some of
the guidance doesn’t cover all availability concepts.  To
further complicate things, there’s a great deal of confusion
about the difference between disaster recovery and high
availability. High availability generally refers to the concept
of keeping an application or service running and available for
use in the event of a failure of part of the infrastructure,
while disaster recovery refers to a process of recovering an
environment that has already failed.  As this article
specifically focuses on high-availability concepts, let’s dive
into SharePoint high-availability concepts first, then look at
some prescriptive guidance for making components in a SharePoint
farm fully redundant and highly available