This may be overly simple, however, I am looking for an explanation as to what exactly the VMkernel is. In some cases (as I understand it) it's defined as an OS that lies on top of the hypervisor (ESXi) that basically manages all resource allocations. Essentially taking the virtualized physical resources provided by ESXi and through VMware products distributing/managing them. In other cases it seems to be very specific to networking (i.e. VMKernel Port Group). In some illustrations, it seems to imply VMkernel is inside the same layer as ESXi. Google is repetitive and not clarifying this for me. I've checked VMware documentation and also curriculum media I have for my classes. Not sure if I'm overlooking something. References would be helpful too. Thank you in advance.
"The VMkernel is a high-performance operating system that runs directly on the ESXihost. The VMkernel manages most of the physical resources on the hardware, including memory, physical processors, storage, and networking controllers. ... TheVMkernel also supports the storage of virtual disks on NFS file systems."
VMware vSphere 5.1 (obviously an outdated version, just an example of some of the information I'm finding)