Hi Tania,
MichaelGebhardt is correct. Release Automation focus on automating the deployment on any given environment. The Continuous Delivery Edition of the product (aka RA CDE) provides planning and collaboration capabilities that allow you to automate the entire delivery process (all the way to production) and, of course, use the fundamental capabilities of RA to perform the actual deployment across the different environments.
Beside that, it help you manage the pipeline (which, hopefully increased, after implementing RA ), prioritize deliveries, tracks conflicts, define and enforce maintenance windows, etc.
Let me know if that make sense.
Uri