How is a container different from a virtual machine?

Short answer

A VM virtualizes hardware and boots a full guest OS with its own kernel — heavy, slow to start, one app per VM is wasteful. A container is just an isolated process sharing the host kernel with its own files, so it starts in milliseconds and packs many apps per host. In practice they are combined: VMs provide hosts, containers pack apps onto them.

Detailed explanation

A VM carries an entire operating system: virtual hardware, guest kernel, drivers, and userland. Isolating a single app that way costs gigabytes and minutes. A container uses Linux namespaces (and cgroups) to give one process its own isolated view — filesystem, networking, process IDs — while sharing the host kernel, so containers stay megabyte-sized and start near-instantly. Trade-off: containers are less strongly isolated than VMs, which is why clouds layer them — provisioned machines are typically VMs, and each VM runs a container runtime hosting many containers, raising utilization and cutting cost. Interview framing: VMs isolate at the hardware level, containers at the process level. Each container should do one thing well (frontend, API, database each in its own container) rather than stuffing a whole stack into one container like a mini-VM.

Official docs:Docker docs: What is a container?

Common mistakes

• Saying containers are just lightweight VMs without mentioning the shared kernel.
• Recommending one container for the whole stack instead of one concern per container.