Virtualization is the idea of creating isolated, self-contained computing environments from a common shared environment. With virtualization we can better divide and manage physical resources and even overcommit those resources depending on the characteristics of the workloads. Virtualization is the foundation concept of a Cloud.
Generally and for practical purposes, there are two main ways of doing virtualization: Containers or Virtual machines. Usually when we say virtualization we are talking about VMs, but containers also share the same fundamental idea of isolation.
While virtual machines use virtualized hardware resources and their own Kernel, containers share the host system’s Kernel and use namespaces, cgroups and other Kernel features to provide isolation. As a result of using less layers of abstractions, containers are more lightweight and less isolated from the host system than VMs. In fact, there’s a huge security market today for security analysis specialized in containers, as it is significantly easier to gain access to the host system from a container than from a VM.
References
https://documentation.ubuntu.com/server/explanation/intro-to/virtualisation/