What is the difference between ContentArea, ContentBlockByKey, and ContentBlockById in AMPscript?

ContentArea(areaNumber) — retrieves a Classic Content Area by its numeric ID from the legacy Classic email content. Largely deprecated in favour of Content Builder.
ContentBlockByKey("externalKey") — retrieves a Content Builder content block by its Customer Key (External Key). This is the preferred method in modern implementations because keys are human-readable and portable across environments.
ContentBlockById(numericId) — retrieves a Content Builder content block by its internal numeric ID. Less portable because IDs differ between business units and environments.
Best practice: always use ContentBlockByKey in production for maintainability. Assign meaningful external keys in Content Builder when creating blocks.