It seems to be an interesting and useful wrapper for Linux Containers, a relatively new feature of the Linux kernel. Docker tries to solve the problem of providing a uniform execution environment across development and production.
In the Python world, virtualenv and pip create a sandbox with specific version numbers for various Python packages, regardless of what may be installed globally on the machine. Docker creates a sandbox of an entire Linux environment, much like a virtual machine, but much lighter-weight than a virtual machine.
Docker instances start in fractions of a second and occupy vastly smaller resources on your laptop or server than a virtual machine would. These days I work on a Macbook and run Linux in a virtual machine, and I'm told it will be practical to simultaneously run multiple Docker instances in the VM. So if you're somebody who's thought it would be fun to write software to run on multiple computers on a network but you haven't had the actual computers available, Docker is for you. I'm interested in Redis as a distributed task queue.
