Docker

VM hypervisors like ESXi, Zen, Hyper-V and KVM are based on simulating virtual hardware which essentially would mean they are loaded (heavy) from the system requirement perspective.  Containers however are a different kettle of fish.  They virtualize the operating system, not the hardware beneath it.  That way they are more efficient than hypervisors in system resources terms.  Instead of virtualizing hardware, the containers reside on top of a single Linux instance.  Lightweight in nature, the containers are a bunch of small, neat tablets or tiny boxes running various applications.

If you could fine tune the container system, you can have as many as five times the number of server application instances as you can while implementing ESXi, Xen or KVM VMs on a hardware.

Docker is built on top of LXC.  The Docker container has its own file system, storage, processor, memory and other resources.  The central contrast between containers and VMs is that while the hypervisor abstracts the entire hardware device the containers just abstract the operating system sitting on top of the hardware device.

This brings to us something that the hypervisors can do that containers cannot.  Hypervisors can bring up instances of different operating systems and kernels while containers cannot do that.  Docker containers, for example, must use the same operating system and kernel.  On the other hand, if the bottomline you are trying to achieve is to make the most out of the server application instances running on the least amount of hardware, Docker containers, or containers in general fit the bill very well since you probably don't care about running multiple operating system VMs.  If you wish to have multiple copies of the same application, then containers will do just fine.

Look at the financial benefits.  Containers can save the Data Center or Cloud provider company millions of dollars per year in terms of hardware costs, footprint and electricity.  For this very reason we find many companies rushing to adopt Docker containers to run in their infrastructure.