Hi Sree,
Depending on the number of users in your company, you could choose between OCS Standard Edition or OCS Enterprise Edition. OCS Standard Edition will support companies with less than 1000 users. This is a single server deployment for OCS.
On the client machines, you will have to rollout Office Communicator.
The OCS_Planning_Guide.doc contains information about which configuration suits best for your company.
HTH
Bernd