Docker and virtual machines are both used to isolate applications and their dependencies from the underlying operating system, but they use different technologies and have different approaches.

Virtual machines (VMs) are essentially emulations of a complete physical machine, including its hardware, operating system, and applications. A hypervisor (such as VMware or VirtualBox) is used to create one or more virtual machines, each of which runs its own operating system and applications. Each VM also has its own allocated resources, such as CPU, memory, and storage.

Docker, on the other hand, uses containerization technology to isolate applications from the underlying system. A container is a lightweight, standalone executable package that includes everything needed to run the application, including the application code, runtime, system tools, libraries, and settings. Docker containers share the kernel of the host operating system and use resources (such as CPU and memory) more efficiently than virtual machines.

In summary, the main differences between Docker and virtual machines are:

Resource usage: Docker containers use fewer resources than virtual machines because they don't require a separate operating system for each container.
Portability: Docker containers are more portable than virtual machines because they can run on any system with Docker installed, regardless of the underlying operating system or hardware.
Startup time: Docker containers can be started much more quickly than virtual machines, which need to boot up a complete operating system.
Isolation: While virtual machines provide complete isolation between different VMs, Docker containers share the host operating system kernel, which may make them less secure in some cases.