A containerizer is a Mesos agent component responsible for launching containers, within which you can run a service. Running services in containers offers a number of benefits, including the ability to isolate tasks from one another and control task resources programmatically.
The Mesos Containerizer is the original container runtime in Mesos. It does not support Docker containers, though it supports a range of isolators that can be composed to create a container. The Mesos Containerizer also does not support container images. Refer to the features matrix to see if the original Mesos containerizer is the correct choice for your task.
The Universal Container Runtime(UCR) extends the Mesos containerizer to support provisioning Docker container images (AppC coming soon). This means that you can use both the Mesos containerizer and other container image types in DC/OS. You can still use the Docker container runtime directly with DC/OS, but the Universal Container Runtime supports running Docker images without depending on the Docker Engine, which allows for better integration with Mesos.