What is REST API?
A REST API is the standard way software systems talk to each other over the web. One system exposes URLs ("endpoints") representing its data — /customers, /orders/123 — and others read or change that data using standard HTTP verbs: GET to fetch, POST to create, PUT to update, DELETE to remove. Responses come as JSON, a simple text format every programming language reads.
APIs are why modern business tools connect: your billing software creates an order, calls the WhatsApp API to message the customer, the payment gateway API to collect money, and the logistics API to book delivery — no human copying data between screens. When evaluating any SaaS, "does it have an API?" determines whether it can integrate into your workflows or will remain an isolated island.