Well fault tolerance as I know
it is reliability i.e the server  is immune to
failures and can react to failures and correct itself
automatically.<br><br>this is done using the familiar aphorism "redundancy
increases reliability". the server actions are duplicated
and exist on say multiple computers as components.
say the same task T is implemented by two different
components A and B. now the server always uses component A
to service the task T. Now if A fails then the
server can use B for implementing T.<br><br>This is
usually done using RMI/CORBA which support distributed
objects.