An app is software designed to perform specific tasks within a cloud computing platform like Salesforce. In Salesforce, apps extend its functionality by allowing customization for specific organizational needs. The main types of apps are native, free, paid, and discounted for nonprofits. Apps optimize Salesforce by enabling features it does not have natively, like allowing unlimited mass emails with customization options. The AppExchange is a site where users can find, review, test, and purchase apps to implement within their Salesforce instance. Some top apps for nonprofits include Vertical Response for mass emails, Demand Tools for data management, and Acteva RSVP for event registration.