what is VMware vCenter Server ?

centralized management utility for all ESX/ESXi hosts and their respective virtual machines

its like windows active directory no exactly the same

vCenter Server is a Windows-based (vCenter Server for Linux is also there),
database-driven application that allows IT administrators to deploy, manage, monitor, automate,
and secure a virtual infrastructure in an almost effortless fashion

The back-end database
(Microsoft SQL Server or Oracle) that vCenter Server uses stores all the data about the hosts and
virtual machines.



Editions of VMware vCenter Server
-------------------------------------------
vCenter Server Essentials (small office)
vCenter Server Standard (all features)
vCenter Server Foundation (same as standard but limited to managingn three ESX/ESXi hosts.)