Case : it is a group of stages , where we configure and understand, how the lifecycle of a casetype(instance of case) goes. Within each stage, a case goes progresses through some required and optional processes.
So a process, is a flow, which exists within a case. There can be casewide supporting process, case wide local actions(which is again a flow), that can be configured on a case.
case is a business requirement, which means work object life cycle,since creation till resolving. But the process is a part of it to full fill a specific requirement say send case to manager for approval and send email on approval etc.
A case is a collection of processes. Generally, the make up of the case itself determines the relationship structure. As the case is reviewed in DCO sessions, the relationships and structure is determined. A few best practices:
A case shouldn't be larger than 7 plus or minus 2 Stages.
A process shouldn't have more than 15 shapes.
For example, In Claim settlement is a case which contains the processes like vehicle check/amount decided to pay for damage/amount approved by insurance company/... which makes up the Claim settlement case.