In general Virtualisation means abstracting software from hardware
OS Level Virtualisation
OS Level Virtualisation Software like VMWare, VirtualBox, HyperV, Citrix etc. uses Virtualisation to separate an Operating System from another one.
For example on a strong server you with Windows Server Edition Operating System you could run Red Hat Linux, Apple Server, etc. in parallel on the same machine.
This allows not only control of an application but also about the Operating System set up and Patch Level for the application.
As this typeo of Virtualisation runs a full Operating system, the requirements for it are higher compared with Containerisation.
Container/Application Level Virtualisation
Opposite to OS Level Virtualisation Container or Application Level Virtualisiation packages Applications, allowing multiple Applications to be deployed in a repreducable way.
This allows less control over the underlying operating system and patchlevel but in return tends to provide higer performance.