Both these technologies is a platform of Cloud Computing. Let's discuss one by one.
AWS- It is a platform where we can perform mostly any kind of process. It can be storage, processing unit, Big Data platform, ML platform etc. For these platform, it changes very less amount, anyone can afford according to their needs. But aws is not opensource, how it works internally, we can't able to know. We can only use it as an user. This makes a difference with OpenStack.
OpenStack-It is also a platform where we can perform the tasks that aws provides. But OpenStack is an opensource platform. We can create our own cloud and customize things according to our needs. We can see how it works internally. You can go through the below link for more information.