In Salesforce, a closed-won opportunity is an opportunity that has been won by the sales team and is no longer active. This usually happens when the customer has signed a contract or made a purchase. Once an opportunity is closed-won, it cannot be reopened. However, Salesforce allows users to create custom fields and statuses for opportunities, so it is possible to create a "closed lost" status for opportunities that were not successfully won. This can be useful for tracking which sales strategies are effective and which need to be improved.