A Case 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.
Work object is an instance of a Case. When you execute or run a Case, you create a Work object.
For ex: You create a Case to get details from a customer. So when Customer A creates his case, Work object 1 is created with details from Customer A. When Customer B creates his case, Work object 2 is created. Case is the same but work objects will be different.