Docker is a lot of things, all rolled up into one so it is difficult to describe without leaving out some detail. What is important to one devops person might be unimportant to another. I have been testing docker for the past few months and there are a couple of things about it that I like quite a bit.
I have to explain a couple of things that I like about it before I get to the one that I really like.
1) It has a repository of very bare bones images for ubuntu, redhat, busybox. Super bare bones, because docker only runs the bare minimum to start with and you build from that.
2) You pull down what you want to work with, and then you figuratively jump into that running image and you can set up that container with what you want it to do.
3) (this is what I really like) That working copy becomes a "diff" of the original base image. You can then save out that working image back to the repository. You can then jump on another machine, and pull down that "diff" image (but you don't even really have to think of it as a "diff", you can just think of it as your new container. docker handles all the magic of it behind the scenes. So if you are familiar with git, it provides a git like interface to managing your server images.
It does a lot more than what I describe above, but it is one of the things I was most impressed with.