clustering is the consupt of high availability where min of
2 servers participate. here the data and the log files are
stored on network storage called to be as nas or san. there
will be a dedicated link between the servers participating
in clustering where the secondary server will be pinging the
primary server to check the heart beet of the primary.
  the disks used for storing the resources are called to be
as Quorum disks. 
   there is a necessity of aver all 5 IP addresses for the
cluster. out of five IP addresses one is called to be as
cluster address and the clients connect to the
cluster/database through this address.
   again there are two types of cluster
1. Active/Active cluster
2. Active/Passive cluster.  u can see more details of this
on the net

when we come to active/passive cluster the secondary node
continuously check the heart beet of the primary node. if
the heart beet stops it assumes that the primary server is
down andd this passive node immediately comes online and
accepts the user connections which in turn is a high
availability solution.

By using the cluster in the organization the database might
be online in most of the cases except that the nas or both
the servers are down.