CloudWatch is mostly used to monitor operational health and performance, but can also provide automation via Rules which respond to state changes. On the other hand, CloudTrail is just used to audit changes to services.
If you’re interested in understanding the differences between the two for the purpose of passing an exam, then you should know a handful of use cases for each, but I think understanding that CloudWatch’s focus on monitoring/automation vs CloudTrail’s (less dynamic) focus on event history auditing is a great base from which to attack exam questions that address this topic