The anatomy of containers
Many people wrongly compare containers to VMs. However, this is a questionable comparison. Containers are not just lightweight VMs. OK then, what is the correct description of a container?
Containers are specially encapsulated and secured processes running on the host system. Containers leverage a lot of features and primitives available on the Linux operating system. The most important ones are namespaces and control groups (cgroups for short). All processes running in containers only share the same Linux kernel of the underlying host operating system. This is fundamentally different from VMs, as each VM contains its own full-blown operating system.
The startup times of a typical container can be measured in milliseconds, while a VM normally needs several seconds to minutes to start up. VMs are meant to be long-living. It is a primary goal of each operations engineer to maximize the uptime of their VMs. Contrary to that, containers are meant to be ephemeral...