Difference between vSphere, ESXi and vCenter - Simplified

VMware Inc. is a software company that develops many suite of software products specially for providing various virtualization solutions.

Virtualization means you are using a big physical computer and running so many computers (Operating systems) simultaneously in it virtually. This machine should be of appropriate capacity in terms of Hard Drive, Memory, Processor etc. The Operating systems running under this big computers are called guest OS

vSphere: vSphere is a software suite that comes under Data Center product. vSphere is like Microsoft Office suite which has many software like MS Office, MS Excel, MS Access and so on. Like Microsoft Office, vSphere is also a software suite that has many software components like vCenter, ESXi, vSphere client and so on. So, the combination of all these software components is vSphere. vSphere is not a particular software that you can install and use. Mark it, vSphere is group (package) of software. Got it?

ESXi, vSphere client and vCenter are components of vSphere.

ESXi: ESXi server is the most important part of vSphere. ESXi is the Virtualization Server.  All the virtual machines or Guest OS are installed on ESXi server. To install, manage and access those virtual servers which sit above (installed and running on) of ESXi server, you will need other part of vSphere suit called vSphere client or vCenter. Now, vSphere client allows administrators to connect to ESXi servers and access or manage virtual machines. vSphere client is installed on the client machine (e.g. Administrator’s laptop). The vSphere client is used from client machine to connect to ESXi server and do management tasks.

So we have vSphere Client for manging the virtual servers. The what is vCenter? Why we need it?

vCenter Server: vCenter server is similar to vSphere client but it’s a server with more power. vCenter server is installed on Windows Server or Linux Server. VMware vCenter server is a centralized management application that lets you manage virtual machines and ESXi hosts centrally. vSphere client is used to access vCenter Server and ultimately manage ESXi servers. vCenter server is compulsory for enterprises to have enterprise features like vMotion, VMware High Availability, VMware Update Manager and VMware Distributed Resource Scheduler (DRS).

(For the time being don't go in details of above terms, we are just trying to understand the VMware jargon)

For example, you can easily clone existing virtual machine in vCenter server. So vCenter is another important part of vSphere package. You have to buy vCenter license separately.

The diagram above shows vSphere suite in a more descriptive way.

vSphere is a product suite (a group of software, you can say).

ESXi is a hypervisor installed on a physical machine. (A hypervisor or virtual machine monitor (VMM) is a piece of computer software, firmware or hardware that creates and runs virtual machines. A computer on which a hypervisor is running one or more virtual machines is defined as a host machine. Each virtual machine is called a guest machine.)

vSphere Client is installed on laptop or desktop PC and is used to access ESXi Server to install and manage virtual machines on ESXi server.

vCenter server is installed as virtual machine on top of ESXi server. vCenter server is a vSphere component which is mostly used in large environment where there are many ESXi server and dozens of virtual machines. The vCenter server is also accessed by vSphere client for management purpose. So, vSphere client is used to access ESXi server directly in small environment. In larger environment, vSphere client is used again to access vCenter server which ultimately manages ESXi server