Its a company that develops a bunch of software and some of it makes containers easier to use. There's also their distro coreos. They are working on rkt which is like docker from what ive seen.
The feeling I get from them is their trying to have their own branded container stack.
IMO their container stack is much better decoupled than Docker's.
In Docker's early days they made a big deal about how a standard container spec could revolutionize software the way the shipping container spec revolutionized the transport of goods.
An independent spec never emerged though. The Docker monolith got bigger and bigger. They threw out standalone mode, so instead of being able to just be a container runtime, Docker basically owned the whole host, and your logging, networking, process monitoring, etc. all had to be done Docker's way.
CoreOS created the App Container spec (https://github.com/appc) to deliver on the promise that Docker started with. They eventually shamed Docker into becoming part of the Open Container Initiative (https://www.opencontainers.org/). Hopefully that means we'll get to a point where application images can truly be shipped anywhere and run anywhere, without having to give any one company control of your whole stack.
As far as I can see rkt has made no effort to implement the opencontainers spec, unlike docker. Meanwhile Docker is implementing all the pieces you are asking for, eg containerd and runc are standalone runtime parts which will break up the monolithic structure, using the spec.
I spend significant time helping the effort (Brandon Philips, CoreOS).
But, today there just isn't anything to implement in rkt. The primary focus of OCI has been a rootfs on-disk and a json file that contains details of cgroups, namespaces, etc. This is fine work to be doing but it doesn't tackle the more difficult and useful work of creating inter-op for developers up the stack who don't care about the internals of containers.
tl;dr Container formats are important to get right. But, today the OCI effort is largely irrelevant for people packaging and shipping their code in a application containers.
The feeling I get from them is their trying to have their own branded container stack.