Hi saurabh,
Parallel approval process allows specifying (upto 25) multiple approvers simultaneously. The approver setting could be set to unanimous or first action. In unanimous parallel approval process, all approvers must approve a request, before it is considered as approved
Its basically how many approvers you set during the approval process configuration
to learn how : http://help.salesforce.com/HTViewHelpDoc?id=approvals_creating_approval_processes.htm&language=en_US
Thank you!