What is a case?

A case is an issue or a request raised by a customer. When a customer raises an issue, you can create a case record in the CRM. 

For example, consider that you are running an e-commerce business. A customer receives a damaged product from you. When they call you to raise a complaint, you log their complaint as a case in the CRM.
In some cases, customers might raise the complaint via email, in which case, Vtiger CRM automatically creates a case record.