What is an API?
An API (application programming interface) is a defined way for one piece of software to request data or actions from another, usually over HTTP with structured JSON.
In more detail
Most business tools publish an API: NetSuite, QuickBooks, Salesforce, HubSpot, Xero. Automations use it to read records, create invoices, update statuses or post journal entries without anyone opening the screen. A few details decide whether an integration works: the authentication method (API key or OAuth), rate limits, pagination, which fields are writable and how errors come back. Documentation and reality sometimes differ, so we test calls against a sandbox first. APIs are more stable than screen scraping, but vendors change and retire them, so keep integrations versioned and monitored.
Example
To post an approved vendor bill, an automation sends the supplier ID, invoice number, dates and coded lines to the accounting system's bill endpoint. The API returns a bill ID, which gets stored on the original email.