What must be registered before making a callout by Visualforce or Apex?

Before making a callout using Visualforce or Apex, it is essential to register a remote site. This registration is crucial because Salesforce enforces security measures to prevent unauthorized external calls from its platform. By defining a remote site, you explicitly whitelist the external endpoint that your application will communicate with, ensuring that the callout can proceed securely.

When a remote site is registered, Salesforce stores the URL and allows Apex code or Visualforce pages to call that specific external service. If an attempt is made to call out to a URL that isn’t registered as a remote site, Salesforce will block the request, thereby upholding the integrity of the system.

In contrast, a connected app relates to integrating with external systems through OAuth and API management and isn’t specifically required for simple callouts. User profiles pertain to user permissions and access levels, which don't directly influence whether a callout can occur. Data records are instances of Salesforce objects and have no direct role in the execution of callouts.