Containers vs virtual machines: a quick comparison
If you've used a virtual machine (VM) before - software like VirtualBox or VMware - you already know one way to isolate software. A container solves a similar problem, but in a much lighter way. Let's compare them.
Virtual machines
A virtual machine runs a full operating system on top of your real one. To run a VM you install a complete guest OS (its own Linux or Windows), which includes its own kernel, system files and everything else. That's powerful, but heavy:
- Each VM takes gigabytes of disk space.
- Each VM needs its own chunk of memory and CPU.
- Starting a VM can take minutes, because a whole operating system has to boot up.
Containers
A container does not ship a full operating system. Instead, all containers on a machine share the host's operating system kernel (the core part of the OS), and each container only adds the files its own app needs.
Because of that sharing, containers are:
- Small - often megabytes instead of gigabytes.
- Fast - they start in seconds or less, because nothing has to "boot".
- Efficient - you can run many containers on the same machine.
When to use a container vs a VM
You don't have to choose one forever - they solve overlapping problems. But as a rule of thumb: if you want to package and run applications quickly and consistently, containers (Docker) are usually the better fit, and that's what this course focuses on.
Here's the mental picture:
- A VM virtualizes the hardware and runs a whole OS on top.
- A container virtualizes just the application, sharing the host's OS.
Next, let's install Docker so we can try this for real.
A detail that surprises people
We said containers share the host's operating system kernel. That's true on Linux. On Windows and macOS, Docker quietly runs a small Linux virtual machine in the background, and your containers share that Linux kernel. So on a Mac or Windows PC you're technically using one lightweight VM to host many containers - you just never manage it yourself. This is why Linux-based images run the same on every operating system.
FAQ
Do containers replace virtual machines?
Not always - they solve overlapping but different problems. Containers are ideal for packaging and running applications; VMs still make sense when you need full operating system isolation or a different OS entirely. Many teams use both.
Are Docker containers as secure as virtual machines?
VMs give stronger isolation because each has its own kernel, while containers share the host kernel. For most application workloads containers are secure enough, especially with good practices (we cover some in the best practices chapter), but a VM's isolation boundary is harder to cross.
Can I run Docker on Windows or macOS if containers need Linux?
Yes. Docker Desktop runs a small Linux virtual machine for you, so Linux-based containers work on Windows and macOS without any extra setup.