An API (Application Programming Interface) is a defined interface — typically over HTTP for modern systems — that lets one software system request data or actions from another. Customer portals depend on APIs to read data from a business’s CRM (customer info), billing system (invoices), file storage (documents), product (usage data), and support platform (tickets).
Common API styles: REST (most prevalent, JSON over HTTP), GraphQL (single endpoint with flexible queries), and SOAP (legacy, XML-based). When evaluating portal platforms, check the API’s depth — can you read AND write all the data you need, or only read? Webhooks for event notifications are also key.