Users of both applications have access to Accounts, Contacts, Opportunities and Activities and most important - to Cases. Why? The reason is that sales people need to have access to the information if their actual customer has opened a Case, what is the status of this Case and if it’s resolved or not. It’s truly important for the salesperson to know if their customers are facing some problems or not before they’re planning to negotiate some subject with them. In such a situation the salesperson is fully armed with the information on the Case status (like “in progress”) and its details and can find the right solution on how to treat the customer.
On the contrary, a customer service agent who is working with the Service Cloud needs to know if there’s some open Opportunity with the customer. Or the Case which he had raised may have an impact on this already opened Opportunity which hypothetically might be harmful for the sales. Plus the sales agent may find a new Opportunity while communicating with the customer .
For example: A service agent talks to the customer about his broken refrigerator and finds out that he intends to buy a new oven which is one of the products that their company sells. The service agent creates a new Opportunity and passes it to the salesperson and lets him/her know about it.
The core idea of Salesforce is to make all the participants of the customer’s interaction stay connected with each other, have the possibility to share the information so that they are fully informed either about the Cases or the Opportunities.