How do you handle callouts in Apex?

Apex callouts send HTTP requests to external systems. Steps:

1. Add the endpoint to Remote Site Settings or use a Named Credential (recommended for authentication).
2. Use HttpRequest, HttpResponse, and Http classes.
3. Callouts cannot be made after DML in the same transaction — use @future(callout=true) or Queueable/Batch to separate them.
4. In test classes, use HttpCalloutMock interface and Test.setMock() to mock HTTP responses.
5. Maximum callout timeout is 120 seconds; 100 callouts per transaction. Named Credentials handle authentication (Basic, OAuth, JWT) declaratively, preventing credentials from being hardcoded in Apex. Always handle HTTP status codes and exceptions (callout timeout, network errors).