vCenter Server and vSphere ESXi hosts available in various versions. VMware release latest version and later they release update for same version. For example vCenter Server 6.0 is the version released, later updated version released as vCenter Server 6.0 Update1, Update1a, Update 1b, Update 2 , Update 2a, etc.
each version has a build number. After you install vCenter server in vSphere client you can this build number. By using this number, you can determine what is the exact version it refers to. Keep ready with this numbers when you open case with VMware for any technical assistance.
While deploying esxi patches you got option to include rollup and updates like (update 2, update 3) as well. Always remember your vCenter Server version should be same or higher version compared to esxi hosts.
VMware release each product with release date and announces its EOL as ( End of General Support and End of Technical Guidance)
These are the very useful links for various vSphere products version & build numbers.
This article tells us about Correlating build numbers and versions of VMware products