What is the difference between Docker and a virtual machine?

Briefly and to the point:

#1. Isolation level

• Virtual machine (VM) - emulates an entire OS (kernel, drivers, services).
• Docker - isolates only the application and its environment, uses the host system's kernel.

#2. Speed

• VM - slow startup (seconds/minutes), heavy.
• Docker - starts in a fraction of a second, lightweight container.

#3. Resource usage

• VM - uses a lot of RAM/CPU, each virtual machine contains an entire OS.
• Docker - containers are "thin", share host resources, minimal overhead.

#4. Flexibility

• VM - suitable when a separate full-fledged system is needed.
• Docker - when many isolated services are needed that are easy to deploy and update.

#5. Key metaphor

• VM - is a separate house (with its own roof, foundation, and utilities).
• Docker - rooms inside one house: isolated, but sharing common walls and infrastructure.

Summary: Docker is lightweight isolation and fast deployment. A virtual machine is heavy but a fully independent environment.