What is the main difference between vRealize Automation and vCloud Director, and can we use them both or we have to use only one of them ?
You can use both of them(vCAC can provision vCD workflows also). At some point of time both will be converged into one. Official Statement "VMware will move forward with a plan to converge vCD functionality into the vSphere and vCloud Automation Center (vCAC) product lines"
Look at the below blog for more information.
Thanks for the clarification