Mac Private Clouds are made possible by a virtualization-providing hypervisor -- a mechanism that allows the physical resources of a bank of our Mac Pro or HP Blade servers to be pooled, so that they can be drawn upon much more dynamically and fluidly.
Orka is MacStadium's proprietary containerization solution for macOS. It effectively splits the difference between the two other options listed, in that it is powerful and easily extensible, but it is also simple enough that the learning curve isn't too steep.
ESXi by VMware
A well trusted provider that offers less expensive, DIY virtualization options. It is worth noting that these options generally require a significant amount of technical expertise to use. The most popular ESXi based solution for MacStadium customers is VSphere, as many enterprise customers use VSphere in other areas and can reuse code as well as have immediate in-house talent to configure their clusters.
Find additional information for comparing the three at choosing your initial cloud.