As we just learned, CI is the ability to quickly integrate many changes to a mainline and is often accompanied by some form of automated testing. The final step in a good CI pipeline is producing some sort of artifact, usually an application, that is ready to be packaged for some deployment.
The practice of taking that artifact and ensuring it is fully packaged and delivered to a pre-production environment is Continuous Delivery.
CD allows get's your application to be "one click" deployed on whatever release schedule your project needs require. With that said, the benefits of CD are better realized if you get your changes into production as soon as possible.
As most of you know the next phase in any DevOps related pipeline is that of Continuous Deployment, which is the removed of that automated "one click" process in favor of an automated release process that places your changes into production without the need for humans!