How does a Docker container differ from a virtual machine?

A Docker container and a virtual machine (VM) are isolation technologies, but they operate differently.

Virtual Machine includes a full operating system running on top of a hypervisor. Each VM has its own OS kernel, libraries, and applications. This provides strong isolation but requires significant resources (RAM, disk space).

Docker container uses the host OS kernel features (such as Linux namespaces and cgroups) to isolate processes. Containers share the OS kernel with the host but have their own filesystem and environment. This makes containers lightweight and quick to start.

Main differences:

• Isolation: VMs are isolated at the hardware virtualization level, containers at the process level.
• Resources: VMs require more resources, containers less.
• Startup speed: Containers start almost instantly, VMs take longer.
• Size: Container images are usually much smaller than VM images.

Example:

# Running a Docker container
docker run -it ubuntu bash

# Running a virtual machine via VirtualBox or VMware requires loading a full OS