A stretched cluster is a deployment model in which two or more virtualization host servers are part of the same logical cluster but are located in separate geographical locations. The vSAN stretched cluster feature enables synchronous replication of data between sites. This feature allows for an entire site failure to be tolerated. It extends the concept of fault domains to data center awareness domains.
vCenter Server is the centralized platform for managing a VMware environment. It is the primary point of management for both server virtualization and vSAN. It is also the enabling technology for advanced capabilities such as vMotion, Distributed Resource Scheduler (DRS), and HA. vCenter scales to enterprise levels where a single vCenter can support up to 2,500 hosts (VxRail nodes) and 30,000 virtual machines. vCenter supports a logical hierarchy of data centers, clusters, and hosts, which allow resources to be segregated by use cases or lines of business. It also allows resources to be moved as needed dynamically. These operations all done from a single interface.