Docker has pushed containers to a new level, making it extremely easy to package and deploy applications within containers. Containers are not new, with Solaris containers and OpenVZ among several containers technologies going back 2005. But Docker has caught on quickly as mentioned by @adrianco. The startup speed is not surprising for containers, the portability is reminiscent of the Java goal to "write once run anywhere". What is truly interesting with Docker is that availability of Docker registries (e.g Docker Hub) to share containers and the potential to change the application deployment workflows.
Rightly so, we should soon see IT move to a docker based application deployment, where developers package their applications and make them available to Ops. Very much like we have been using war files. Embracing a DevOps mindset/culture should be easier with Docker. Where it becomes truly interesting is when we start thinking about an infrastructure whose sole purpose is to run containers. We can envision a bare operating system with a single goal to manage docker based services. This should make sys admin life easier.
The role of the Cloud with Docker
While the buzz around Docker has been truly amazing and a community has grown over night, some may think that this signals the end of the cloud. I think it is far from the truth as Docker may indeed become the killer app of the cloud.
A IaaS layer is what is: an infrastructure orchestration layer, while Docker and its ecosystem will become the application orchestration layer.
The question then becomes: How do I run Docker in the cloud ? And there is a straightforward answer: Just install Docker in your cloud templates. Whether on AWS or GCE or Azure or your private cloud, you can prepare linux based templates that provide Docker support. If you are aiming for the bare operating system whose sole purpose is to run Docker then the new CoreOS linux distribution might be your best pick. CoreOS provides rolling upgrades of the kernel, systemd based services, a distributed key value store (i.e etcd) and a distributed service scheduling system (i.e fleet)