vCenter Server 6 has some fundamental architectural changes compared to vCenter Server Server 5.5. The multitude of components that existed in vCenter Server 5.x has been consolidated in vCenter Server 6 to have only two components vCenter Management Server and Platform Services Controller, formerly vCenter Server Single Sign-On.
The Platform Services Controller (PSC) provides a set of common infrastructure services encompassing
Single Sign-On (SSO)
The vCenter Management Server consolidates all the other components such as Inventory Service & Web Client services along with its traditional management components. The vCenter Server components can be typically deployed in with either embedded or external PSC. Care should be taken to understand the critical differences between the two deployment models. Once deployed one cannot move from one mode to another in this version.
More details:- http://www.vmware.com/files/pdf/techpaper/vmware-vcenter-server6-deployment-guide.pdf