Docker provides Platform-as-a-Service (PaaS) through OS-level virtualization. This form of virtualization allows for the deployment of software in packages called containers. Containers are isolated from each other and bundle their own software, libraries, and configuration files; they can communicate with each other through well-defined channels. OS-level virtualization is lightweight compared to other forms of virtualization because it does not require a hypervisor to create virtual machines. Instead, the Docker Engine enables the containers to run directly within the host machine's operating system but with separate namespaces, which is why it's considered OS-level.