What is an API?
An API, or application programming interface, is a documented way for one piece of software to request data or actions from another, such as asking a CRM to create a contact or an accounting system to list unpaid invoices.
Think of an API as a service counter with a published menu. The menu lists what you can ask for, such as "give me today's appointments" or "create an invoice for this customer," and the format the request must follow. Software that knows the menu can place orders without a person clicking through screens. Most modern cloud software, from QuickBooks Online to Clio, publishes an API.
For a business owner, the practical question is whether a product's API covers what you need. Some APIs only allow reading data, some limit how many requests you can make, and some lock key features behind higher subscription tiers. When evaluating software, ask the vendor for API documentation and check that the specific records you care about, like jobs, matters or invoices, can be read and written.
APIs are what make integration platforms, AI tools and custom dashboards possible. They also create responsibility. Anything connected through an API can see or change the data it is allowed to access, so permissions should be as narrow as practical, access should be tied to a company account, and connections should be reviewed when staff or vendors change.
Example
For example, a roofing contractor wants its estimating spreadsheet to stop being retyped into its field service software. A developer checks the field service vendor's API, confirms it can create estimates with line items, and builds a small connector. The estimator still reviews each estimate in the field service app before it goes to the homeowner.